Journal of Petrology | Volume 43 | Number 12 | Pages 2171-2190 | 2002
© Oxford University Press 2002
Mafic Magma Intraplating: Anatexis and Hybridization in Arc Crust, Bindal Batholith, Norway
1DEPARTMENT OF GEOSCIENCES, TEXAS TECH UNIVERSITY, LUBBOCK, TX 79409-1053, USA
2DEPARTMENT OF GEOLOGY AND MINERAL RESOURCES ENGINEERING, NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY, N-7491, TRONDHEIM, NORWAY
3GEOLOGICAL SURVEY OF NORWAY, N-7491, TRONDHEIM, NORWAY
4GEOLOGICAL SURVEY OF NORWAY, MINERALOGICAL–GEOLOGICAL MUSEUM, 0562 OSLO, NORWAY
Received July 17, 2001; Revised typescript accepted May 2, 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
The dioritic Velfjord plutons (
448 Ma) were emplaced into regional migmatitic metapelitic and metacarbonate rocks at mid-crustal levels, corresponding to pressures of 700 MPa. Exhumation to 400 MPa began while the migmatites were in a partly molten state. With increasing proximity to the plutons, regional stromatic migmatites change to diatexite, and diatexitic dikes are common within 500 m of the contacts. We interpret these relationships to indicate that heat from the plutons resulted in contact migmatization in a zone up to 1 km wide. Typical residual mineralogy in the diatexites is plagioclase + quartz + biotite + garnet + sillimanite ± K-feldspar, consistent with biotite dehydration melting. Pod- and dike-like leucosomes consist of two types: earlier high-K (granitic) ones with mineral assemblages identical to the migmatites and later low-K (tonalitic) ones in which sillimanite is sparse and garnet absent. The high-K leucosome magmas can be explained by biotite dehydration melting at 700 MPa. Within the aureole, mafic magmas were locally injected into, and hybridized with, the diatexites and the high-K leucosome magmas. In contrast, the low-K leucosomes are thought to result from local, late-stage remelting of H2O-saturated diatexite. The H2O-rich fluid was probably released from intergranular melt trapped in the diatexites during exhumation and solidification. Distinctive porphyritic ‘contact granites’ are common at pluton contacts. Although the mineral assemblage of these granites is identical to that of the diatexites, their isotopic compositions are distinct, with 
Nd and 
18O in the migmatites from -7·6 to -9·6 and from +10·9
to +13·5, and in the contact granites from -5·2 to -7·5 and from +9·6 to +12·3, respectively. Thus, the contact granites could have a source that is isotopically distinct from, but mineralogically similar to the diatexites, or they could result from mixing of magma similar to the high-K leucosomes with dioritic magmas. Mass balance calculations are consistent with the latter interpretation, with proportions of granitic to dioritic magmas from 7:1 to 7:3. Emplacement and solidification of the dioritic plutons provided zones of structural anisotropy along which high-K leucosome magmas and contact granite magmas collected. These magmas were injected by additional dioritic magma and further hybridized. Because the solidi of the plutons were several hundred degrees higher than that of the granitic magmas, the pluton walls acted as long-lived, hot, rigid surfaces along which magmas collected and migrated. KEY WORDS: migmatite; contact melting; hybridization; magma transport; Caledonian
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
The importance of crustal melting in arcs has been widely accepted since the development of isotopic fingerprinting methods. Controversy still exists concerning the volumetric importance of crustal melts and the mechanisms by which crustal melts interact with mantle-derived mafic magmas. In fact, identification of magmas that are purely crustal in origin is commonly difficult [see Collins (1998) and Patiño Douce (1999); and Chappell (1996) and White et al. (1999) for contrasting views]. Patiño Douce (1999) compared experimental data with data for a variety of natural granitic suites and concluded that most arc granites result from some degree of hybridization between crustal- and mantle-derived magmas.
A particularly vigorous discussion has arisen concerning mechanisms of crustal melting and ways in which magma is extracted, transported, and accumulated into large plutons (e.g. Clemens & Mawer, 1992; Paterson & Fowler, 1993; Brown et al., 1995b; Sawyer, 1996, 1998; Clemens, 1998; Solar & Brown, 2001). This is at least in part due to difficulties in the establishment of a direct relationship between zones of crustal melting (migmatites) and granitic plutons (e.g. Sawyer, 1996, 1998; Solar & Brown, 2001). For example: Which processes are responsible for separation of magma from its source and removal of entrained residual solids [see review by Brown et al. (1995a)]? Can a lithologic unit be repeatedly melted to form distinct magmas, or does a single melting event make the source too refractory to produce anything but ultra-high-T melts (e.g. Beard et al., 1993)? Does crustal melting in an arc setting depend on underplating or intraplating of mafic magmas (Hildreth & Moorbath, 1988; Huppert & Sparks, 1988; Bergantz, 1989) or can it be related entirely to an elevated geothermal gradient? What influence does intrusion of mafic magmas have on the segregation, migration, and petrology of anatectic granites?
The Bindal Batholith is a continental arc-like batholith that probably formed near the Laurentian side of the Caledonian orogen (Roberts, 1988). In several locations, migmatites are intimately associated with mafic plutons, and the inference has been made that mafic magmatism triggered or enhanced crustal melting (e.g. Barnes et al., 1992). We report on a suite of migmatites formed by contact melting during emplacement of dioritic plutons, on the granitic magmas produced, and on possible relationships with felsic magmatism in the Batholith. We also suggest that structural anisotropy formed by emplacement of mafic plutons into migmatitic terranes enhanced the collection and transport of anatectic magmas and focused the hybridization process.
|
GEOLOGIC SETTING |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
The Bindal Batholith (Fig. 1) is the northernmost of two Ordovician–Silurian batholiths in north–central Norway; the other is the Smøla–Hitra Batholith west of Trondheim (Gautneb & Roberts, 1989). The host rocks of the Bindal Batholith are part of the Helgeland Nappe Complex (HNC, Fig. 1) (Kollung, 1967; Myrland, 1972; Gustavson, 1978; Thorsnes & Løseth, 1991), which is the structurally highest unit in the Uppermost Allochthon of the Caledonian nappe stack in central Scandinavia (Stephens et al., 1985). The HNC is composite and consists of Late Proterozoic, medium- to high-grade pelitic, semi-pelitic, calc-silicate, and calcareous rocks interleaved with Early Ordovician ophiolite complexes and their unconformable cover sequences (medium- to low-grade sandstones, conglomerates, and calc-silicate rocks). Amalgamation of terranes within the HNC took place in Ordovician time, before the Middle Ordovician to Early Silurian emplacement of the Batholith. The final eastward translation (modern coordinates) of the HNC, including the Bindal Batholith, across lower-grade rocks of the Upper Allochthon took place in the Late Silurian to Devonian Scandian stage of the Caledonian orogeny.
|
The earliest magmatism in the Batholith consisted of sparse, mildly to strongly peraluminous alkali–calcic granites [e.g. tourmaline granite in Velfjord (Fig. 2a), biotite granite near Brønnøysund (Fig. 1), and ‘anatectic granite’ of Vega (Fig. 1)]. U–Pb (zircon) ages range from 481 to 468 Ma (Yoshinobu et al., 2001), which is coeval with regional migmatization in the Velfjord area. Emplacement of voluminous gabbroic to granitic plutons began at 448 Ma, with intrusion of the Velfjord and Andalshatten plutons (Fig. 1; Nordgulen et al., 1993; Pedersen et al., 1999). This magmatism, which was predominantly alkali–calcic to calc-alkalic, continued to 430 Ma (Nordgulen & Schouenborg, 1990; Nordulen et al., 1993). Pb, Sr, and Nd isotopic data (Nordgulen & Sundvoll, 1992; Birkeland et al., 1993) require metasedimentary and meta-igneous crustal sources, as well as a mantle component, in post-448 Ma parts of the Batholith, but pre-468 Ma plutons have isotopic compositions consistent with crustal sources.
|
The Velfjord plutons consist of three large bodies (Hillstadfjellet, Akset–Drevli, and Sausfjellet) and two smaller ones (Svarthopen and Aunet) (Fig. 2a; Barnes et al., 1992). The Sausfjellet pluton is dioritic whereas the Akset–Drevli pluton consists of Fe-rich gabbro and diorite. The Hillstadfjellet pluton was emplaced in two stages: an early gabbroic Stage 1 and a later, more voluminous, monzonitic Stage 2 (Fig. 2a), with compositional range from diorite to quartz monzonite. The gabbro–diorite body west of Sørfjord (Fig. 2a) is herein named the Svarthopen pluton [‘uralitic gabbro’ of Myrland (1972)]. This pluton is crosscut and locally mingled with porphyritic granites that we refer to as ‘contact granites’ in a later section. Similar granites crop out in arcuate masses adjacent to the Svarthopen body (Fig. 2a).
The wall-rocks of all the Velfjord plutons consist of intercalated high-grade marbles and migmatitic pelitic and quartzofeldspathic gneisses. The Sr and oxygen isotopic compositions of the marbles suggest a Neoproterozoic age (Trønnes, 1994; Trønnes & Sundvoll, 1995). Each lithology constitutes mappable units of kilometre scale (Myrland, 1972), but variations within a unit can encompass all three rock types. Among the migmatitic rocks, only the pelitic gneisses show evidence of appreciable magma production during contact metamorphism, therefore our discussion will focus on them.
The high-grade metamorphic unit is structurally overlain by a package of Ordovician(?) ultramafic rocks and non-migmatitic metasedimentary rocks (Fig. 2a). Thorsnes & Løseth (1991) interpreted the metasedimentary rocks as a cover sequence unconformably overlying the ultramafic rocks (e.g. at Heggefjord; Fig. 2a). A similar stratigraphic sequence is recognized in a number of thrust sheets within the HNC (Nordgulen, 2000; Heldal, 2001).
The contact between this structurally higher ultramafic + cover unit and the lower migmatitic rocks was interpreted as a thrust fault by Thorsnes & Løseth (1991). However, it places younger (Ordovician?), lower-grade, non-migmatitic rocks over older (Neoproterozoic), higher-grade, migmatitic rocks. This relationship, and top-to-the-east shear sense indicators show that final displacement on the fault was in a normal sense (Barnes & Prestvik, 2000).
Pelitic gneisses of the lower, high-grade unit
The pelitic gneisses vary from micaceous to quartzofeldspathic (Electronic Appendix 1, which may be downloaded from the Journal of Petrology website, at http://www.petrology.oupjournals.org.). Probable protoliths include shales, wackes, and sparse feldspathic arenites. Most of the unit is migmatitic, but non-migmatitic medium-grained schists are present. These schists are typically calcite rich and some contain calcic amphibole. As such, their CaO-rich bulk composition prevented partial melting. The migmatitic gneisses underwent two distinct periods of anatexis. The first was a regional event (regional migmatites) associated with early peraluminous magmatism in the Batholith, whereas the second was local and related to emplacement of the Velfjord plutons (contact migmatites).
Regional migmatites
The regional pelitic migmatites are predominantly layered (stromatic) migmatites with millimeter- to centimeter-scale layers of alternating quartzofeldspathic (leucosome) and mica-rich material. They typically lack melanosomes. Less abundant vein migmatites display discontinuous centimeter-scale leucosomes. Diatexites (migmatites that lack internal structure) are sparse; they are intercalated with stromatic and vein migmatites. All types are foliated, with foliation defined by aligned biotite and sillimanite ± staurolite ± muscovite (Table 1; Electronic Appendix 1). They also all contain calc-silicate pods that range from a few centimeters to several meters in length. The various migmatite types are intercalated with sparse marble lenses and pods of amphibolite. Quartz veins and felsic dikes are common. The dikes vary from garnet two-mica monzogranite and biotite pegmatite to biotite hornblende tonalite; leucotonalite and leucomonzogranite are most common.
|
Sausfjellet contact zone
Within 1 km of the Sausfjellet pluton (Fig. 2b), the pelitic rocks are primarily diatexitic. Distinct blocks of stromatic migmatite can be identified but are discontinuous along strike. Calc-silicate blocks (schollen) are typically broken, with tonalitic (‘low-K’) leucosomes filling fractures (e.g. Fig. 3f). In this zone, folds are locally intruded by leucosomes parallel to axial planes. Such features are not observed outside the aureole of the pluton.
|
Less than 500 m from the pluton, the migmatitic rocks consist of well-foliated to discontinuously banded diatexite (essentially schlieric banding defined by sillimanite ± biotite) that is cut by isotropic diatexites. The latter units truncate foliation in the former, have weak or no foliation, and typically have hypidiomorphic granular texture. These cross-cutting, isotropic diatexites commonly contain biotite-rich clots, which are thought to be remnants of disrupted biotite-rich compositional bands. Locally, the isotropic diatexites enclose blocks of foliated diatexite, as well as pods of massive quartz, and calc-silicate schollen with late-stage tonalitic leucosomes in fractures, as described above.
The pluton–diatexite contact is locally cut by dikes of feldspar-phyric garnet-bearing biotite quartz monzonite to granite (Fig. 3d). As will be shown below, such rocks are characteristic of these contacts. Therefore, for the sake of simplicity and despite their range of modal variation, the dikes are referred to as ‘contact granites’ (Table 1; Electronic Appendix 1). Diatexite dikes intrude the southwestern margin of the Sausfjellet pluton. These dikes have limited lateral extent and they commonly brecciate the host gabbro and diorite. The contact migmatites, contact granites, and pluton are cut by dikes of medium-grained, equigranular leucocratic monzogranite and tonalite (Electronic Appendix 1).
Akset–Drevli contact zone
A similar change from stromatic to diatexitic migmatites can be seen along the southern margin of the Akset–Drevli pluton, although the transition occurs over a distance of <500 m. Furthermore, at its southern contact, the Akset–Drevli pluton is separated from migmatites by a band of contact granite of 100–200 m width (Fig. 2c) identical to the contact granite adjacent to the Sausfjellet pluton. Mafic dikes intruded the contact diatexites while the diatexites were still molten. This resulted in formation of mafic pillows in, and magma mingling with, leucosome magma-rich zones of the diatexites (Fig. 3h). Hybridization during mingling formed garnet-bearing mafic enclaves in variably granitic to diatexitic dikes.
Hillstadfjellet contact zone
The contact between the Hillstadfjellet pluton and migmatites is exposed along Sørfjord and Heggefjord (Fig. 2a). Leucosome-rich diatexite with blocks of stromatic migmatite and calc-silicate is typically in contact with the pluton. Diatexite dikes intrude the margins of the pluton and hybridization between the diatexite and the pluton locally produced hornblende-rich garnet-bearing diorite.
Medium- to fine-grained leucosomes are generally leucogranite (‘high-K leucosomes’); but sparse leucotonalite (low-K leucosomes; Table 1 and Electronic Appendix 1) cuts the high-K leucosomes. Stringers of leucogranite are connected to larger pods of leucosome (Fig. 3a), which are elongate parallel to foliation and interfinger with the diatexite along foliation planes. Some leucosomes are intimately folded with the host diatexite (Fig. 3b) but the same leucosomes contain folia and disrupted fragments of the host migmatite. Along Heggefjord, contact granite dikes intrude, and are locally hybridized with, Stage 1 diorites (location N40.91, Fig. 2a), whereas along Sørfjord, contact granite is sparse, is typically elongate in the plane of foliation, and is pinched out along foliation-parallel shear zones.
The diatexites, leucosomes, and contact granite were intruded by fine- to coarse-grained dioritic dikes (Fig. 3c). Along strike these dikes are broken into angular to cuspate mafic enclaves; some dikes have cuspate margins against contact granite. Such features are identical to mingled synplutonic dikes that are common in many granitic plutons (see Barnes et al., 1986) and indicate that mafic magmas were emplaced into the migmatitic margins of the plutons while the migmatites were partly molten.
Late-stage, near-solidus deformation formed a strong foliation parallel to the intrusive contact. The intensity of the foliation is greater in the contact migmatites than in the pluton; the migmatites locally have S–C fabrics with down-to-the-east sense of shear.
Ages of migmatization
The proximity of contact granites to the Velfjord plutons suggests that the granites resulted from contact anatexis associated with pluton emplacement (Barnes et al., 1992). This idea was tested with U–Pb (SHRIMP) dating of zircon from contact granite sample N19.91 collected adjacent to the eastern contact of the Akset–Drevli pluton (Fig. 2a). Details have been presented by Yoshinobu et al. (2001). Zircons from sample N19.91 yield a bimodal age distribution, with a cluster of five ages with mean 467·8 ± 4·9 Ma and a cluster of six ages with mean 447·1 ± 3·7 Ma (ages at 95% confidence interval). The 467·8 Ma age is within the age range of the older anatectic granites of the Bindal Batholith (see above). It is interpreted to represent the age of regional migmatization in the Velfjord area. The 447·1 Ma age is identical to that of the adjacent pluton (448 Ma; Pedersen et al., 1999) and is consistent with formation of the granitic body by contact anatexis.
|
GEOCHEMISTRY |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Elemental data
Analytical methods are described in the Appendix. Sample locations are shown in Fig. 2. Major and trace element compositions of granitic rocks, migmatites, and host rocks are given in Electronic Appendix 2, which may be downloaded from the Journal of Petrology website, at http://www.petrology.oupjournals.org.
Figure 4 gives a normative classification (Barker, 1979) of the leucosomes, contact granites, and diatexitic dikes; it illustrates the range of rock types among the contact granite group, from granite to tonalite. Other geochemical distinctions between the various migmatites, non-migmatitic metasedimentary rocks, and granitic rocks are illustrated in Fig. 5, which shows variations of K2O, CaO, Mg/(Mg + Fe), and normative corundum as a function of SiO2 content. The tie-lines in Fig. 5a and b connect compositions of leucosomes and adjacent diatexites. The single analyzed diatexite–high-K leucosome pair was sampled in the southern contact zone of the Akset–Drevli pluton.
|
|
Compositional differences between the two types of leucosome (low vs high K) and the contact granites are shown in Fig. 5. The low-K leucosomes have K2O contents <2·4 wt %, CaO contents from 2·5 to 4·6 wt % and Mg/(Mg + Fe) >0·43. In contrast, the high-K leucosomes have K2O from 2·5 to 5·8 wt %, CaO contents <1·5 wt % and Mg/(Mg + Fe) between 0·3 and 0·4. All of the leucosomes have SiO2 contents >68 wt %, with the high-K leucosomes between 73 and 80 wt % SiO2.
The contact granites display a wider SiO2 range than any other granite group (Fig. 5) and all but two of them have SiO2 contents <69 wt %. The two high-SiO2 exceptions consistently plot in or near the field of high-K leucosomes. The contact granites show decreasing CaO with increasing SiO2 and variable K2O contents (Fig. 5). Although characterized by considerable scatter, it is apparent that the contact granite trend is not collinear with the compositional trends of the migmatites (diatexites and bulk stromatic migmatites).
All of the pelitic migmatites have SiO2 <65 wt %. On average, the stromatic migmatites display higher bulk SiO2 than the diatexites, from 62 to 65 wt %. Most samples are low in CaO (<1·5%; Fig. 5b); these samples show negative correlation between CaO and Al2O3 (not shown). The few high-CaO, low-Al2O3 migmatites are similar in composition to non-migmatitic rocks from the area, except that the latter have higher Mg/(Mg + Fe) (Fig. 5c).
The migmatites are conspicuous in their high normative corundum (Cor) contents relative to the granitic rocks and non-migmatitic metapelites (Fig. 5d). The migmatites plot on a steep trend nearly perpendicular to the trend of the contact granites. Stromatic migmatites are typically less Cor rich than the diatexites. Diatexitic dikes have high Cor values and plot with the diatexites rather than the granites. If, as field relations suggest, the contact diatexites formed by contact melting of regional stromatic migmatites, then their overall higher normative Cor suggests that melt was lost from the diatexites, leaving them with refractory compositions.
Isotopic data
Samples for oxygen, Sr, and Nd isotopic analysis were chosen to be representative of the mineral assemblages of each group and to encompass their ranges of major element compositions. Analytical methods are presented in the Appendix. One of the plotted contact granite analyses is taken from Nordgulen & Sundvoll (1992) and Birkeland et al. (1993).
Among the leucosomes and migmatitic rocks, Nd(448 Ma) ranges from -5·3 to -9·6 (Table 2); however, all but one sample have values more negative than -7·6 (Fig. 6a). The contact granites range from -5·2 to -7·5. Although there is minor overlap between the contact granites and the migmatites, the contact granites have generally higher Nd values. For the purposes of comparison, the range of Nd for the Velfjord plutons is +0·3 to -3·8 (average -2·0; Fig. 6a; C. G. Barnes, T. Prestvik & B. Sundvoll, unpublished data, 1994) and Nd values in metapelites from the structurally higher non-migmatitic terrane average -11·3.
|
|
Oxygen isotope ratios for whole rocks and quartz separates are given in Table 2. For samples in which both were measured, quartz is 1 higher than whole-rock values, which is typical of quartz–whole-rock fractionation at high T (e.g. Taylor & Sheppard, 1986). Whole-rock 18O values for stromatic migmatites and diatexites range from +10·9 to +13·5 (Table 2; Fig. 6b). This range of values is nearly identical to that of the leucosomes. The contact granites have 18O values from +9·6 to +12·3, which are slightly lower than in the migmatites. In contrast, the oxygen isotopic compositions of the Velfjord plutons range from +5·9 to +9·8 and average +7·5.
Initial Sr isotope ratios (calculated for 448 Ma) are available only for the Velfjord plutons and the contact granites. The former range from 0·7057 to 0·7070 (Nordgulen & Sundvoll, 1992; C. G. Barnes, T. Prestvik & B. Sundvoll, unpublished data, 1994) and the latter from 0·7123 to 0·7223 (Fig. 7; Table 2). The contact granite values are within the range of metasedimentary rocks from the region reported by Nordgulen & Sundvoll (1992). However, Nordgulen & Sundvoll (1992) did not analyze samples of the high-grade migmatites exposed adjacent to the Velfjord plutons.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Models for development of the contact migmatites and the origins of the contact granites must explain the following:
- close to the Velfjord plutons, the migmatites become predominantly diatexitic and isotropic, cross-cutting diatexites are present;
- the contact granites are proximal to the Velfjord plutons, locally intrude and hybridize with them, yet are more intensely penetratively deformed than the adjacent diorites;
- the mineral assemblages in the diatexites, high-K leucosomes, and contact granites are similar (garnet, biotite, plagioclase, quartz ± alkali feldspar ± sillimanite), whereas the low-K leucosomes typically lack garnet and alkali feldspar;
- the compositional trend of the contact granites is distinct from that of the migmatites;
- the most siliceous contact granites are compositionally similar to the high-K leucosomes;
- the contact granites have a U–Pb age identical to that of the Velfjord plutons;
- contact granites are coeval with the high-K leucosomes, but are isotopically distinct from them;
- low-K leucosomes are the youngest leucosome type.
Our first consideration is whether development of diatexitic migmatites is related to emplacement of the Velfjord dioritic magmas. This close association was originally interpreted to result from contact melting caused by heat from the plutonic magmas (Barnes et al., 1992). The alternative is that the pluton-ward transition from stromatic to diatexitic migmatites and the presence of isotropic, cross-cutting diatexites adjacent to the plutons is fortuitous. We find the latter explanation unlikely in view of the absence of voluminous diatexites distal from mafic plutons.
Our second consideration is the origin of the leucosomes and the contact granites; specifically, whether their source was the local diatexitic migmatites. A local origin for the high-K leucosomes is consistent with their mineral assemblage and the broad overlap of oxygen and Nd isotope compositions; however, the data do not rule out melt migration within the migmatite by vertical or lateral flow. This is particularly true in view of the evidence for decompression while the migmatites were still partly molten (Barnes & Prestvik, 2000). A local origin for the low-K leucosomes is likewise consistent with the isotopic data and the prevalence of such leucosomes in boudin necks and filling fractures in schollen blocks. If such is the case, then an origin of high-K and low-K leucosomes from the same source must be explained.
Finally, the isotope data show that the contact granites cannot be simple partial melts of the local migmatites. Thus, the contact granites could represent magmas from a deeper, isotopically distinct source, or they could represent the effects of local partial melting combined with hybridization (magma mixing). In this regard, it is noteworthy that feldspar-phyric plutonic rocks are common in the Bindal Batholith (Nordgulen, 1992). These plutons are characterized by K-feldspar phenocrysts, by Sr contents that reach 800 ppm at 67% SiO2, and by initial 87Sr/86Sr <0·710 (Fig. 7; Nordgulen & Sundvoll, 1992). In contrast, the Velfjord area contact granites contain K-feldspar and plagioclase phenocrysts (Electronic Appendix 1), commonly carry accessory garnet and sillimanite, have Sr contents no higher than 320 ppm, and have initial 87Sr/86Sr between 0·7123 to 0·7223 (Fig. 7). These features, combined with the gradation between coarse leucosomes and contact granites, suggest a local origin for the contact granites. In the following sections, we discuss the origin of the leucosomes and of the most siliceous contact granites. We then consider possible petrogenetic links between these high-SiO2 rocks and the range of contact granite compositions. This is followed by a discussion of possible implications for transfer of anatectic magmas, magma mixing in a migmatitic realm, and the significance of such activity in the Bindal Batholith.
Origin of the leucosomes
Recent experimental studies on partial melting of pelitic and semipelitic rocks [summarized by Patiño Douce (1999)] permit direct comparison of observed leucosome compositions with experimentally produced glasses. Any such comparison must consider the possibility that leucosomes do not represent melt compositions but instead are partial cumulates (e.g. Brown et al., 1995b; Sawyer, 1998) or contain appreciable amounts of residual minerals. Figure 8 is a plot of molar K, (Na + Ca), and (Fe + Mg + Ti) for the granites and migmatites, along with compositions of constituent minerals. The compositional ranges of experimentally produced glasses are shown for biotite dehydration melting of metawacke, muscovite dehydration melting of biotite muscovite schist, and H2O-saturated melting of metawacke. (See the caption of Fig. 8 for data sources.)
|
Three of the high-K leucosomes and the most siliceous (i.e. low Fe + Mg + Ti) members of the contact granites (seven samples) plot in or near the fields for dehydration melting of metawacke or biotite muscovite schist (Fig. 8). The experiments show that such melts are in equilibrium with the assemblage quartz + K-feldspar + plagioclase + biotite ± sillimanite ± garnet; the assemblage present in the high-K leucosomes and contact granites. At a pressure of 700 MPa, the melting temperatures necessary to form these residual minerals are in the 750–800°C range (Barnes & Prestvik, 2000).
Previously, it was noted that the contact diatexites are more refractory than the regional stromatic migmatites and this relationship is apparent when average compositions are plotted (Fig. 9a). The refractory nature of the contact diatexites is consistent with loss of melt similar in composition to the high-K leucosomes. This idea was tested with linear least-squares mass balance calculations (Table 3), which show that 22–24% melting of an average regional stromatic migmatite (Table 3) produces a magma of SiO2-rich high-K leucosome or contact granite type, leaving a residue similar to the average diatexite composition (Fig. 9a). The sum of squares of residuals (
r2) is 0·5. The goodness of fit is improved if 2–4% feldspars are fractionated from the assemblage (r2 0·2).
|
|
Four of the low-K leucosome compositions plot to the left of the field for H2O-saturated partial melts of metawacke (Fig. 8), and two other samples plot at much higher proportions of (Fe + Mg + Ti). The lack of overlap with the field of experimentally produced, H2O-saturated, tonalitic glasses (Fig. 8) is due to the slightly higher (Fe + Mg + Ti) contents and somewhat lower K contents in the leucosomes.
One explanation for such compositions is accumulation of plagioclase ± ferromagnesian minerals (cumulates or residual phases) in a leucosome magma. Furthermore, the simplest explanation is to call on such accumulation from magmas of high-K leucosome type. However, plagioclase in the low-K leucosomes is more calcic (An62–41) than plagioclase in the high-K leucosomes (An35–22), is distinctively oscillatory zoned, and has a characteristic blocky habit (Electronic Appendix 1). Furthermore, some high-K leucosomes contain K-feldspar phenocrysts; therefore, K-feldspar should be a cumulus phase in at least some of the low-K leucosomes. We conclude that even if the low-K leucosomes are cumulates, they are not cumulates from magmas of high-K leucosome type, but from a magma capable of precipitating calcic plagioclase.
Two types of leucosome from a single source?
The isotopic evidence is consistent with a local origin for both the high-K and the low-K leucosomes. Furthermore, field relations indicate that both leucosome types formed during the contact melting event, and that the high-K leucosomes formed before the low-K ones. If true, this relationship requires early, higher-T, dehydration melting to form the high-K leucosomes (750–800°C; Barnes & Prestvik, 2000), followed by later, lower-T, H2O-saturated melting to form the low-K leucosomes (as low as 675°C; Conrad et al., 1988). This sequence is unusual in that one would expect H2O to be consumed at the beginning of the melting process.
A resolution to this contradiction can be found in the nature of the migmatitic source rocks. In spite of their residual chemical compositions, most of the contact diatexites retained significant melt fractions, either because melt could not completely escape or because melt migrated into the diatexite (Sawyer, 1998). In either case, the contact diatexites represent refractory, yet melt-bearing compositions. Because H2O was concentrated in the melts during biotite dehydration melting, the trapped melts would exsolve H2O-rich fluid during decompression and crystallization (Spear et al., 1999). The common retrograde development of muscovite in many of the diatexites attests to this exsolution. We suggest that transport and local collection of exsolved H2O-rich fluid resulted in local, H2O-saturated remelting of the refractory contact diatexites to produce low-K tonalitic leucosomes (Fig. 9c). This explanation is consistent with the higher Mg/(Mg + Fe) values and CaO contents of the low-K leucosomes, because they formed by melting of a somewhat more refractory source. A refractory source is also consistent with the relatively small volume of these leucosomes.
Compositional variation of the contact granites
The contact granites are puzzling because they are isotopically distinct from the local migmatites but show chemical, structural, textural, and geographic evidence for a local origin (see above). In Fig. 8, the contact granites define a broad trend that extends away from mica-dehydration minimum melt compositions toward the (Na + Ca)–(Fe + Mg + Ti) join. The trend cannot result solely from variable separation of residual minerals because many compositional plots lack linearity between the contact granites and the diatexitic migmatites (Figs 5, 6 and 8). This does not preclude the presence of residual phases in the contact granites, but indicates that separation of such phases is not the only cause of compositional variation.
The contact granites have a phenocryst assemblage (plagioclase, alkali feldspar, biotite, garnet ± quartz) that is stable in a narrow T range of 750–800°C (Barnes & Prestvik, 2000), yet they span a large compositional range. This suggests that the compositional variations must result from a process other than variable degrees of partial melting.
The elemental and isotopic compositions of the contact granites can be modeled by variable hybridization of mafic magmas with local, high-K leucosome-like granitic magmas, combined with accumulation of phenocrysts and residual minerals. A simple mixing scenario was tested with major element mass balance calculations (Table 3). If leucocratic contact granitic magma is mixed with ‘average’ Akset–Drevli or Sausfjellet pluton compositions (Fig. 9b), then mixtures of 20–30% mafic end-member can crudely fit the data (r2 from 1·4 to 2·3). These simple mixing models assume that the mixing end-members and products were melts; the models fail to account for accumulated phenocryst and residual crystals. Mass balance calculations that couple magma mixing with crystal accumulation yield mass percentages of felsic melt, mafic melt, and accumulated crystals in the range: 70% felsic melt, 7–9% mafic melt, 17% accumulated feldspars, 3% accumulated garnet, and <1% accumulated apatite and ilmenite. The r2 values for these models range from 0·03 to 0·05.
Isotopic mass balance (simple mixing) for oxygen and Nd suggests that contact granites can result from mixing proportions of 85% magma of high-K leucosome type and 15% mafic magma. The discrepancy between major element and isotopic models could result from the fact that some of the cumulate feldspars and garnet carry isotopic signatures of the granitic end-member. Therefore, we interpret the contact granites to be hybrids of mafic magmas and local, high-K granitic melts, modified by accumulation of phenocrysts ± residual phases.
A consequence of this interpretation is that hybridization need not occur in a single, well-defined locality, but can occur wherever sufficient mafic magma is available for mixing. At Velfjord, hybridization occurred in mafic dikes that cut the migmatites (Fig. 3h) and in contact granite zones adjacent to the Velfjord plutons (Fig. 3c). Because hybridization was local, non-uniform elemental and isotopic trends would be expected. Homogeneous hybridization would require convective mixing of the contact granite magmas.
Migration of the contact granite magmas
The contact granites are characterized by small proportions of residual material and by relatively low-T mineral assemblages. As noted above, these factors suggest that the location of the contact granites immediately adjacent to the Velfjord plutons is not a function of raised T and consequent greater degrees of melting. This is in contrast to the increase in proportion of leucosomes and increased mobility of the diatexites, which clearly are related to contact melting effects. Instead, we suggest that the contact granites occur at the boundary between diatexite and mafic plutons because of rheological gradients imposed by juxtaposition of the mechanically strong, pyroxene ± melt-bearing plutons and weak, quartz + feldspar + melt-bearing diatexites.
Because biotite dehydration melting results in a weak positive volume change (Rushmer, 2001), disruption of the migmatite crystal framework is critical to development of significant accumulations of magma. Collection of leucosomes and contact granite intrusions occurred via magma migration to regions of lower differential stress and anisotropic effects associated with solidification of the large, mafic plutons. At the outcrop scale, leucosomes collected in strain shadows such as boudin hinges (Collins & Sawyer, 1998) and adjacent to resistant blocks (Fig. 3f and g). During transport of the larger contact granite bodies, the mafic Velfjord plutons probably behaved as resistant blocks (‘mega-schollen’) because they reached their solidus at much higher temperatures than the surrounding migmatites. [The pluton solidi are as high as 900°C for the Sausfjellet pluton on the basis of pyroxene closure temperatures and as low as 750°C for the Hillstadfjellet pluton on the basis of hornblende–plagioclase thermometry (Barnes & Prestvik, 2000). The contact granite wet solidus is 650°C.]
Contact granite magma may have migrated to strain shadows protected by the resistant mafic plutons (e.g. southwestern margin of the Akset–Drevli pluton, Fig. 2a and c). However, because contact granites also occur within regions that are perpendicular to the maximum principal shortening direction (e.g. southeastern margin of the Akset–Drevli pluton, Fig. 2a), other mechanisms must have been active. We suggest that the temperature and rheological gradient across the pluton–diatexite boundary provided an anisotropy into which the contact granite magmas were focused as dikes (Fig. 10). Such anisotropic effects could allow magmas to migrate in dikes that are not in the optimum orientation relative to the maximum principal shortening and extension directions (i.e. parallel to the z-axis and perpendicular to the x-axis of the finite strain ellipsoid; Lucas & St.-Onge, 1995). High-K leucosome magmas and related hybrids could migrate into these regions and collect to form pockets of granitic magma. Further contraction of the plutons during solidification and cooling could promote or maintain a strain field that was optimal for localization of magmas along the pluton–diatexite boundary. Collection of the contact granite magma at pluton margins allowed for longer cooling, acted as traps for additional mafic input and subsequent hybridization (Fig. 10), and permitted growth and accumulation of feldspar phenocrysts.
|
If the plutons acted as mega-schollen, then their rigid walls may have provided conduits for contact granite magmas to migrate upward, away from the zone of melting (Fig. 10). This form of magma transport would provide a pre-heated channel for magma flow and a long-lived zone into which anatectic magmas could migrate. We suggest that the large contact granite bodies adjacent to the Svarthopen pluton (Fig. 2a) were emplaced by such a process and that they collected along the margins of a larger mafic body at slightly deeper structural levels.
Diatexites as open magmatic systems
The relationship between migmatites and granitic plutons has been widely discussed [e.g. see reviews by Brown (1994), Brown et al. (1995b) and Clemens (1998)]. Stromatic migmatites have clear mechanisms and evidence of melt transfer, such as veins and concentrations of leucosome material (e.g. Brown et al., 1995b). In contrast, diatexites commonly lack field evidence for melt transfer and loss. On the basis of elemental concentrations, Sawyer (1998), Milord et al. (2001) and Solar & Brown (2001) have shown that some diatexites are refractory relative to their protolith and therefore must have lost a melt component.
This relationship is apparent in the Velfjord contact diatexites. In fact, the most refractory compositions are generally those of the mobile contact diatexites (i.e. dike-like and isotropic cross-cutting diatexites). This implies either that the melt component migrated to adjacent diatexites by permeable flow or migrated away from the local zone of migmatization. The former explanation is unlikely because few, if any, diatexites have compositions that indicate addition of a melt component. The latter explanation is consistent with collection of anatectic magmas adjacent to the dioritic plutons.
The contact diatexites were open systems not only with regard to melt loss, but also with regard to addition of mafic magmas. The presence of mingled, hybrid zones within the diatexites and along the margins of plutons (e.g. Hillstadfjellet and Svarthopen), and the isotopic evidence for mixing in the contact granites, suggests that dioritic magmas were able to invade and mix with the contact diatexitic mush. Such hybrid zones can be viewed as small-scale analogs of the lower-crustal MASH zones of Hildreth & Moorbath (1988) in which wholesale injection, melting, and hybridization of the crust occur.
|
RELATIONSHIP TO OTHER GRANITES OF THE BINDAL BATHOLITH |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Figure 7a is a comparison of Sr contents in the contact granites with other granitic rocks in the Bindal Batholith. The contact granites belong to a group of plutons (e.g. the Gaupen, Andalshatten, Heilhornet, and Bindalseidet plutons, and the Alsten Massif, Fig. 1) whose Sr contents are low relative to the rest of the Batholith (Nordgulen, 1992). Dated plutons in this group have U–Pb ages
444 Ma (Nordgulen et al., 1993). Low Sr contents also characterize the older (481–468 Ma) anatectic Vega pluton and the tourmaline granite west of Velfjord (Fig. 1). In contrast, the few dated plutons in the ‘high-Sr’ field have U–Pb ages 
443 Ma (Nordgulen et al., 1993). Many of the low-Sr plutons are also distinct in their higher initial 87Sr/86Sr compared with the rest of the Batholith (Fig. 7b) and at least one (the Gaupen pluton) has contact granites in its aureole. The presence of anatectic granites, the lower Sr contents, and higher initial 87Sr/86Sr among the older plutons in the Batholith suggest that crustal anatexis of pelites played a prominent role early in the development of the Batholith. This suggests that (1) pelitic crustal source(s) or mixing end-members of the lower-Sr plutons underwent anatexis before 444 Ma, but were not important after that time, or that (2) the pelitic source rocks were geographically restricted to certain parts of the Batholith. The latter possibility could result from pre- or syn-Batholith tectonic imbrication of source terranes. In either case, additional thermobarometric, geochronologic, and isotopic data are necessary for a thorough understanding of the positions of crustal magma sources and the contributions of advected (magmatic) heat, regional metamorphism, and tectonic imbrication before or during formation of the Batholith.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Mid-crustal melting in the Helgeland Nappe Complex is at least locally related to emplacement of mafic (dioritic) magmas. Following regional high-grade metamorphism from 481 to 468 Ma, emplacement of mafic plutons at 448–447 Ma caused mica dehydration melting of pelitic and semipelitic rocks. This contact melting event resulted in a transition from regional stromatic migmatites to contact diatexitic migmatites with proximity to the plutons. Leucosomes produced during this event were granitic; they tended to collect adjacent to the plutons. Dioritic dikes emplaced into the partly molten contact diatexites and related leucosomes hybridized with them to form ‘contact granite’ magmas. Crystallization of the migmatites resulted in release of H2O-rich fluid, which caused local, volumetrically minor, H2O-saturated remelting to produce tonalitic leucosomes.
The higher solidi of the dioritic plutons compared with the migmatitic and contact granite magmas meant that the plutons acted as mega-schollen in their migmatitic host. As a consequence, the plutonic contacts locally acted as anisotropic boundaries where contact granite magmas collected. Once in place, the contact granites trapped additional influxes of mafic magma, and hybridized with them. The hot, rigid, vertically extensive plutonic contacts also accommodated upward transport of the granitic magmas.
Thus, emplacement of mafic plutons into mid-crustal rocks can result in additional melting of those rocks, can provide a locus for melt collection and upward transport, and can provide sites of hybridization. Such crustal ‘intraplating’ may provide a mechanism to explain the hybrid nature of many ‘anatectic’ granites (e.g. Elburg & Nicholls, 1995; Collins, 1998).
|
APPENDIX: ANALYTICAL METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Major elements and Rb, Sr, Zr, Y, Nb, Ba, Cr, Cu, and Zn were determined by X-ray fluorescence (XRF) at the Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology. Loss on ignition (LOI) was first determined by the weight loss of adsorbed water by heating 2–3 g sample powder in a porcelain cup at 120°C overnight (M1). Subsequently, the weight loss was determined by heating the samples in the same porcelain cup to 900°C in a furnace for 5 h (M2). The reported LOI values are M2 – M1.
XRF analyses were made using a Philips PW1480 XRF system. Major elements were determined on glass beads, which were made by fusing 0·5000 g ignited rock powder with 5·0000 g LiBO2–LiB4O7 (66:34).
International standards were used for calibration. The accuracy, the deviation of the ‘true’ value, as determined on the international standards BCR-2 and GSP-2, is in general better than 3% with the exception of MnO (10%). Precision for the major elements is typically better than 1·5% and better than 4% for the trace element. Detection limits for trace elements are 2 ppm for Zn, Cu, and Co; 3 ppm for Zr, Y, Sr, Rb, Ni, and Th; 4 ppm for Pb; 5 ppm for Cr and V; and 10 ppm for Ba.
Oxygen was liberated from silicates using the BrF5 method of Clayton & Mayeda (1963) and converted to CO2 by passage over a hot graphite rod. Oxygen isotopic ratios were obtained on a VG SIRA-12 dual-inlet mass spectrometer. All values are relative to V-SMOW. Silicate analyses are precise to ±0·2. The average and standard deviation obtained for NBS-28 is 9·45 ± 0·2 (standard error is ±0·02).
The Nd isotope ratios were determined by isotope dilution methods at the Mineralogical–Geological Museum, University of Oslo, and using a VG Isotope VG354 TIMS instrument. The analytical procedure was identical to that reported by Mearns (1986). During the analytical work the JM-reference standard gave a value of 143Nd/144Nd = 0·51111 ± 0·00005. Errors quoted are 2
standard errors.
|
ACKNOWLEDGEMENTS |
|---|
We thank M. Barnes for her considerable assistance in the field and laboratory, and S. Swapp, G. J. DeHaas, and I. Vokes for help in the laboratory. Thorough reviews by M. Brown, T. Rushmer, and K. Skjerlie helped us clarify our arguments; however, they are not implicated in our conclusions. Partial support for this work came from NSF grant EAR9814280.
|
FOOTNOTES |
|---|
*Corresponding author. E-mail: Cal.Barnes{at}ttu.edu
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONGEOLOGIC SETTINGGEOCHEMISTRYDISCUSSIONRELATIONSHIP TO OTHER GRANITES...CONCLUSIONSAPPENDIX: ANALYTICAL METHODSREFERENCES |
|---|
Barker, F. (1979). Trondhjemite: definition, environment and hypotheses of origin. In: Barker, F. (ed.) Trondhjemites, Dacites and Related Rocks. Amsterdam: Elsevier, pp. 1–12.
Barnes, C. G. & Prestvik, T. (2000). Conditions of pluton emplacement and anatexis in the Caledonian Bindal Batholith, north–central Norway. Norsk Geologisk Tidsskrift 80, 259–274.[Web of Science]
Barnes, C. G., Allen, C. M. & Saleeby, J. B. (1986). Open- and closed-system characteristics of a tilted plutonic system, Klamath Mountains, California. Journal of Geophysical Research 91, 6073–6090.
Barnes, C. G., Prestvik, T., Nordgulen, Ø. & Barnes, M. A. (1992). Geology of three dioritic plutons in Velfjord, Nordland. Norges Geologiske Undersøkelse Bulletin 423, 41–54.
Beard, J. S., Abitz, R. J. & Lofgren, G. E. (1993). Experimental melting of crustal xenoliths from Kilbourne Hole, New Mexico and implications for the contamination and genesis of magmas. Contributions to Mineralogy and Petrology 115, 88–102.[Web of Science]
Bergantz, G. W. (1989). Underplating and partial melting: implications for melt generation and extraction. Science 245, 1093–1094.
Birkeland, A., Nordgulen, Ø., Cumming, G. L. & Bjørlykke, A. (1993). Pb–Nd–Sr isotopic constraints on the origin of the Caledonian Bindal Batholith, central Norway. Lithos 29, 257–271.[Web of Science]
Brown, M. (1994). The generation, segregation, ascent and emplacement of granite magma: the maigmatite-to-crustally-derived granite connection in thickened orogens. Earth-Science Reviews 36, 83–130.
Brown, M., Averkin, Y. A. & McLellan, E. L. (1995a). Melt segregation in migmatites. Journal of Geophysical Research 100, 15655–15679.
Brown, M., Rushmer, T. & Sawyer, E. W. (1995b). Introduction to special section: mechanisms and consequences of melt segregation from crustal protoliths. Journal of Geophysical Research 100, 15551–15563.
Chappell, B. W. (1996). Compositional variation within granite suites of the Lachlan Fold Belt: its causes and implications for the physical state of granite magma. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 159–170.[Web of Science]
Clayton, R. & Mayeda, T. (1963). The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochimica et Cosmochimica Acta 27, 43–52.[Web of Science]
Clemens, J. (1998). Observations on the origins and ascent mechanisms of granitic magmas. Journal of the Geological Society, London 155, 843–851.
Clemens, J. D. & Mawer, C. K. (1992). Granitic magma transport by fracture propagation. Tectonophysics 204, 339–360.[Web of Science]
Collins, W. J. & Sawyer, E. W. (1996). Pervasive granitoid magma transfer through the lower-middle crust during non-coaxial compressional deformation. Journal of Metamorphic Geology 14, 565–579.[Web of Science]
Collins, W. (1998). Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: implications for crustal architecture and tectonic models. Australian Journal of Earth Sciences 45, 483–500.[Web of Science]
Conrad, W. K., Nicholls, I. A. & Wall, V. J. (1988). Water-saturated and -undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb: evidence for the origin of silicic magmas in the Taupo Volcanic Zone, New Zealand, and other occurrences. Journal of Petrology 29, 765–803.
Elburg, M. A. & Nicholls, I. A. (1995). Origin of microgranitoid enclaves in the S-type Wilson’s Promontory Batholith, Victoria: evidence for magma mixing. Australian Journal of Earth Sciences 42, 423–435.[Web of Science]
Gautneb, H. & Roberts, D. (1989). Geology and petrochemistry of the Smøla–Hitra Batholith, Central Norway. Norges Geologiske Undersøkelse Bulletin 416, 1–24.
Gustavson, M. (1978). Caledonides of north–central Norway. Geological Survey of Canada, Paper 78-13, 25–30.
Heldal, T. (2001). Ordovician stratigraphy in the western Helgeland Nappe Complex in the Brønnøysund area, north–central Norway. Norges Geologiske Undersøkelse Bulletin 438, 47–61.
Hildreth, W. & Moorbath, S. (1988). Crustal contributions to arc magmatism in the Andes of central Chile. Contributions to Mineralogy and Petrology 98, 455–489.[Web of Science]
Huppert, H. E. & Sparks, R. S. J. (1988). The generation of granitic magmas by intrusion of basalt into continental crust. Journal of Petrology 29, 599–624.
Kollung, S. (1967). Geologiske undersøkelser i det sørlige Helgeland og nordlige Namdal. Norges Geologiske Undersøkelse Bulletin 254, 95 pp.
Lucas, S. B. & St-Onge, M. R. (1995). Syn-tectonic magmatism and the development of compositional layering, Ungava orogen (northern Quebec, Canada). Journal of Structural Geology 17, 475–492.[Web of Science]
Mearns, E. W. (1986). Sm–Nd ages from Norwegian garnet peridotite. Lithos 19, 269–278.[Web of Science]
Milord, I., Sawyer, E. W. & Brown, M. (2001). Formation of diatexite migmatite and granite magma during anatexis of semi-pelitic metasedimentary rocks: an example from St. Malo, France. Journal of Petrology 42, 487–505.
Montel, J.-M. & Vielzeuf, D. (1997). Partial melting of metagreywackes, Part II. Compositions of minerals and melts. Contributions to Mineralogy and Petrology 128, 176–196.[Web of Science]
Myrland, R. (1972). Velfjord. Beskrivelse til det berggrunns-geologiske gradteigskart I 18—1:100,000, 274. Trondheim: Norges Geologiske Undersøkelse, 30 pp.
Nordgulen, Ø. (1992). A summary of the petrography and geochemistry of the Bindal Batholith. Norges Geologiske Undersøkelse Report 92.111, 103 pp.
Nordgulen, Ø. (2000). Structural evolution of the Helgeland Nappe Complex, central Norwegian Caledonides. Geonytt 27, 127–128.
Nordgulen, Ø. & Schouenborg, B. (1990). The Caledonian Heilhornet pluton, north–central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides. Journal of the Geological Society, London 147, 439–450.
Nordgulen, Ø. & Sundvoll, B. (1992). Strontium isotope composition of the Bindal Batholith, Central Norwegian Caledonides. Norges Geologiske Undersøkelse Bulletin 423, 19–39.
Nordgulen, Ø., Bickford, M., Nissen, A. & Wortman, G. (1993). U–Pb zircon ages from the Bindal Batholith, and the tectonic history of the Helgeland Nappe Complex, Scandinavian Caledonides. Journal of the Geological Society, London 150, 771–783.
Paterson, S. R. & Fowler, T. K., Jr (1993). Re-examining pluton emplacement processes. Journal of Structural Geology 15, 191–206.[Web of Science]
Patiño Douce, A. E. (1999). What do experiments tell us about the relative contributions of crust and mantle to the origin of granitic magmas? In: Castro, A., Fernandez, C. & Vigneresse, J. (eds) Understanding Granites: Integrating New and Classical Techniques. Geological Society, London, Special Publications 168, 55–75.
Patiño-Douce, A. E. & Beard, J. S. (1996). Effects of P, fO2 and Mg/Fe ratio on dehydration melting of model metagreywackes. Journal of Petrology 37, 999–1024.
Patiño Douce, A. & Harris, N. (1998). Experimental constraints on Himalayan anatexis. Journal of Petrology 39, 689–710.[Web of Science]
Patiño Douce, A. E. & McCarthy, T. C. (1998). Melting of crustal rocks during continental collision and subduction. In: Hacker, B. R. & Liou, J. G. (eds) When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-pressure Rocks. Dordrecht: Kluwer Academic, pp. 27–55.
Pedersen, R.-B., Nordgulen, Ø., Barnes, C. G., Prestvik, T. & Barnes, M. (1999). U–Pb dates from dioritic and granitic rocks in Velfjord, north–central Norway. Geonytt 26, 81.
Pickering, J. & Johnston, A. (1998). Fluid-absent melting behavior of a two-mica metapelite: experimental constraints on the origin of Black Hills granite. Journal of Petrology 39, 1787–1804.[Web of Science]
Roberts, D. (1988). The terrane concept and the Scandinavian Caledonides: a synthesis. Norges Geologiske Undersøkelse Bulletin 413, 93–99.
Rushmer, T. (2001). Volume change during partial melting reactions: implications for melt extraction, melt geochemistry and crustal rheology. In: Teyssier, C., Vanderhaeghe, O. & Edwards, M. A. (eds) Partial Melting and the Flow of Orogens. Tectonophysics 342, 399–405.
Sawyer, E. W. (1996). Melt segregation and magma flow in migmatites: implications for the generation of granite magmas. Transactions of the Royal Society of Edinburgh, Earth Sciences 87, 85–94.[Web of Science]
Sawyer, E. W. (1998). Formation and evolution of granitic magmas during crustal reworking: the significance of diatexites. Journal of Petrology 39, 1147–1167.[Web of Science]
Solar, G. S. & Brown, M. (2001). Petrogenesis of migmatites in Maine, USA: possible source of peraluminous leucogranite in plutons? Journal of Petrology 42, 789–823.
Spear, F., Kohn, M. & Cheney, J. (1999). P–T paths from anatectic pelites. Contributions to Mineralogy and Petrology 134, 17–32.[Web of Science]
Stephens, M. B., Gustavson, M., Ramberg, I. B. & Zachrisson, E. (1985). The Caledonides of central–north Scandinavia—a tectonostratigraphic overview. In: Gee, D. G. & Sturt, B. A. (eds) The Caledonide Orogen—Scandinavia and Related Areas. Chichester: Wiley, pp. 135–162.
Taylor, H. P., Jr & Sheppard, S. M. F. (1986). Igneous rocks: I. Processes of isotopic fractionation and isotope systematics. In: Valley, J. W., Taylor, H. P., Jr & O’Neil, J. R. (eds) Stable Isotopes in High Temperature Geologic Processes. Mineralogical Society of America, Reviews in Mineralogy 16, 227–271.
Thorsnes, T. & Løseth, H. (1991). Tectonostratigraphy in the Velfjord–Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides. Norges Geologiske Undersøkelse Bulletin 421, 1–18.
Trønnes, R. G. (1994). Marmorforekomster i Midt-Norge: geologi, isotopgeokjemi og industrimineralpotensiale. Norge Geologiske Undersøkelse Report 94.042, 21 pp.
Trønnes, R. G. & Sundvoll, B. (1995). Isotopic composition, deposition ages and environments of Central Norwegian Caledonian marbles. Norges Geologiske Undersøkelse Bulletin 427, 44–48.
Vielzeuf, D. & Montel, J. M. (1994). Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships. Contributions to Mineralogy and Petrology 117, 375–393.[Web of Science]
White, A., Chappell, B. & Wyborn, D. (1999). Application of the restite model to the Deddick Granodiorite and its enclaves—a reinterpretation of the observations and data of Maas et al. (1997). Journal of Petrology 40, 413–421.[Web of Science]
Yoshinobu, A. S., Barnes, C. G., Nordgulen, Ø., Dumond, G. & Fanning, M. (2001). Ordovician magmatism and deformation in the Uppermost Allochthon, central Norway: an orphan of the Taconic orogeny? Geological Society of America, Abstracts with Programs 33, A204.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  C. G. Barnes, T. Prestvik, Y. Li, L. McCulloch, A. S. Yoshinobu, and C. D. Frost Growth and zoning of the Hortavaer intrusive complex, a layered alkaline pluton in the Norwegian Caledonides Geosphere, June 1, 2009; 5(3): 286 - 301. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. E. Otamendi, M. N. Ducea, A. M. Tibaldi, G. W. Bergantz, J. D. de la Rosa, and G. I. Vujovich Generation of Tonalitic and Dioritic Magmas by Coupled Partial Melting of Gabbroic and Metasedimentary Rocks within the Deep Crust of the Famatinian Magmatic Arc, Argentina J. Petrology, May 8, 2009; (2009) egp022v1. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. S. Yoshinobu and C. G. Barnes Is stoping a volumetrically significant pluton emplacement process?: Discussion Geological Society of America Bulletin, July 1, 2008; 120(7-8): 1080 - 1081. [Full Text] [PDF]
|
|
|
|
 |
|
 D. Roberts, O. Nordgulen, and V. Melezhik The Uppermost Allochthon in the Scandinavian Caledonides: From a Laurentian ancestry through Taconian orogeny to Scandian crustal growth on Baltica Geological Society of America Memoirs, January 1, 2007; 200(0): 357 - 377. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. G. Barnes, E. V. Mars, S. Swapp, and C. D. Frost Petrology and geochemistry of the Middle Jurassic Ironside Mountain batholith: Evolution of potassic magmas in a primitive arc setting Geological Society of America Special Papers, January 1, 2006; 410(0): 199 - 221. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Dumond, A. S. Yoshinobu, and C. G. Barnes Midcrustal emplacement of the Sausfjellet pluton, central Norway: Ductile flow, stoping, and in situ assimilation Geological Society of America Bulletin, March 1, 2005; 117(3-4): 383 - 395. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||