Journal of Petrology - current issue

Garnetites of the Cardigan Pluton, New Hampshire: Evidence for Peritectic Garnet Entrainment and Implications for Source Rock Compositions

Dorais, M. J., Pett, T. K., Tubrett, M. — Wed, 18 Nov 2009 06:20:39 PST

The Cardigan Pluton, located in southwestern New Hampshire, USA, is a strongly peraluminous granodiorite that contains distinctive, meter-sized pods consisting of 50–70 modal % garnet (+sillimanite + biotite + plagioclase + quartz). The presence of fibrolitic mats and flat, unzoned major and trace-element garnet profiles provide evidence for prograde metamorphism and single-stage garnet growth from biotite dehydration melting. Melt-depleted, bulk-rock compositions indicate that the garnetites are either fragments of restite or melt-depleted xenoliths. Comparison of the Nd and Sr isotopic compositions of the garnetites and the Cardigan granitic rocks indicates an equilibrium relationship as required in the restite model. Additionally, plagioclase and garnet compositions are the same in the garnetites and the most mafic host rocks, also permissive of a restite origin for the garnetites. Temperature and pressure calculated using garnet–biotite thermometry and garnet–aluminum silicate–quartz–plagioclase (GASP) barometry yield estimates of 800°C and 6–7 kbar. These temperatures are probably lower than the peak melting temperature because major element modeling subtracting a granodiorite composition from an average calc-pelite metasediment requires greater than 45% melting to generate garnetite sample CP-23G. Such high melting percentages require temperatures of ~900°C, near the biotite-out curve. We infer that the heat required for such high amounts of melting was obtained from asthenospheric upwelling and basaltic underplating. Inherited zircons with 600 Ma U–Pb ages suggest that the Cardigan Pluton does not have a Laurentian source, consistent with thermobarometric calculations that place the depth of melting below the décollement between the basement and the Central Maine Trough metasediments. A peri-Gondwanan basement source is inferred. Calculated ascent rates of >1000 km/yr allowed preservation of restite in the Cardigan Pluton whereas slower rates in other peraluminous plutons could account for the paucity of restite in most peraluminous plutons.

Continental Reworking during Overprinting Orogenic Events, Southern Prince Charles Mountains, East Antarctica

Phillips, G., Kelsey, D. E., Corvino, A. F., Dutch, R. A. — Wed, 18 Nov 2009 06:20:40 PST

In situ electron microprobe monazite dating and mineral equilibria modelling of amphibolite–granulite-facies metapelites from the southern Prince Charles Mountains, East Antarctica has been carried out to unravel the P–T conditions, spatial extent and structural style of two overprinting orogenic records. This study shows that: (1) rocks of the northern Palaeoproterozoic Lambert Complex were pervasively reworked at peak conditions (6·5–7·1 kbar and 790–810°C) during the Early Neoproterozoic Rayner orogenic event; (2) rocks of the southern Lambert Complex experienced pervasive deformation and metamorphism at peak conditions (5·8–6·1 kbar and 625–635°C) during Early Palaeozoic Prydz orogenic activity; (3) in regions of the Lambert Complex reworked during the Rayner orogenic event, Prydz-aged orogenesis was highly localized. The distribution of orogenic activity pertaining to the Rayner and Prydz orogenic events in the southern Prince Charles Mountains can be attributed to (1) the development of a southward directed (current coordinates) orogenic front that propagated from an Early Neoproterozoic collision between India and Antarctica, and (2) rock fertility (i.e. availability of free fluid) during Early Palaeozoic intraplate orogenesis that was driven by far-field stresses generated by a collision of India–Antarctica with the Mawson Craton.

The High P-T Stability of Hydroxyl-apatite in Natural and Simplified MORB--an Experimental Study to 15 GPa with Implications for Transport and Storage of Phosphorus and Halogens in Subduction Zones

Konzett, J., Frost, D. J. — Wed, 18 Nov 2009 06:20:40 PST

Experiments have been conducted in the range 3–15 GPa and 850–1800°C to investigate the P–T stability field of OH-apatite in an average mid-ocean ridge basalt (MORB) and a model Mg-basalt, to study the compositional evolution of apatite and its breakdown products and the partitioning of P between phosphates and silicates. In the bulk compositions investigated OH-apatite is stable to <7·5 GPa at 950°C in a typical eclogite assemblage garnet + omphacite + SiO₂ + TiO₂. This is ~5 GPa below the breakdown P of pure OH-apatite. The high-P breakdown product is tuite [-Ca₃(PO₄)₂]. Both apatite and tuite are stable in a wide range of subduction zone T regimes but not along an average mantle adiabat. This precludes apatite or tuite stability in the asthenospheric mantle. Apatite may be stable in cold continental lithosphere (40 mW/m²) but is restricted to P < ~4–5 GPa. The apatite breakdown reaction is an important limit for the crust–mantle transport of Cl in subduction zones and can contribute to the Cl depletion of subducted cust. Both apatite and tuite are important storage sites for large ion lithophile elements (LILE) and rare earth elements (REE), therefore apatite breakdown does not greatly affect LILE or REE transport in subduction zones. In an eclogite assemblage only garnet can accommodate significant P. In the presence of apatite or tuite, P₂O₅ contents in garnet range from ~0·2 to 0·6 wt % between 3 and 11 GPa and increase to ~0·8 wt % at 15 GPa in the absence of a detectable phosphate phase. The P-storage capacity of clinopyroxene is limited to ~250 ppm. Because of the extreme preference of P for the garnet structure, virtually the entire P budget of subducted MORB will be locked up in garnet well into the lower mantle provided fO₂ is high enough to prevent the stability of a metal phase.

In Situ U-Pb and Trace Element Analysis of Accessory Minerals in the Kiruna District, Norrbotten, Sweden: New Constraints on the Timing and Origin of Mineralization

Smith, M. P., Storey, C. D., Jeffries, T. E., Ryan, C. — Wed, 18 Nov 2009 06:20:40 PST

Northern Norrbotten, Sweden is a key part of Baltic Shield and provides a record of magmatic, tectonic and related, superimposed, Fe oxide–apatite and iron oxide–copper–gold (IOCG) mineralization, during the Svecokarelian orogeny. Titanite and allanite from a range of mineral deposits in the area have been analysed for U–Pb isotope systematics and trace element chemistry using laser ablation quadrupole inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses of a single sample from the regional scapolite–albite alteration give an age of 1903 ± 8 Ma (2) and may be contemporaneous with the early stages of Fe mineralization (1890–1870 Ma). Analyses of titanite and allanite from undeformed IOCG deposits indicate initial alteration at 1862 ± 16 Ma. In many deposits subsequent metamorphic effects reset titanite isotope systematics from 1790 to 1800 Ma, resulting in a spread of U–Pb isotope analyses along concordia. In some instances core regions may record evidence of early thermal events at around 2050 Ma. Titanite and allanite from deformed IOCG deposits on major shear zones record ages from 1785 ± 21 Ma to 1777 ± 20 Ma, corresponding to deformation, metamorphism and secondary hydrothermal alteration as a result of late orogenic movements. The lack of intracrystalline variations in titanite and allanite trace element chemistry suggests that hydrothermal fluid chemistry and metal source were the main controls on mineral trace element chemistry. Titanite from undeformed Fe oxide–apatite and IOCG deposits is typically light rare earth element (LREE) enriched, and shows low U/Th ratios and low Ni in both intermediate to acid and basic volcanic-hosted deposits. This is consistent with a granitic source for metals. Minor variations in trace element patterns are consistent with the influence of aqueous complex formation on relative REE solubility. Deposits related to the Nautanen Deformation Zone have relatively heavy REE (HREE)-enriched titanite, and LREE-depleted allanite, with high U/Th ratios and elevated Ni contents, consistent with leaching of metals from the local basic volcanic rocks. All hydrothermal titanites are high field strength element enriched (Nb, Ta, Zr) indicating their transport as a result of either high salinities or high F contents, or both. The data overall support models of IOCG-type mineralization as a result of regional circulation of saline hydrothermal fluids in association with major crustal structures, with at least some metallic components derived from the granitoid rocks of the area. All the deposits here show evidence of subsequent metamorphism, although penetrative fabrics are restricted to regional-scale deformation zones.

Magma Recharge and Crystal Mush Rejuvenation Associated with Early Post-collapse Upper Basin Member Rhyolites, Yellowstone Caldera, Wyoming

Girard, G., Stix, J. — Wed, 18 Nov 2009 06:20:40 PST

The Upper Basin Member rhyolites are the oldest known post-collapse rhyolites of the third Yellowstone caldera. They were erupted near the caldera's resurgent domes between 516 ± 7 and 473 ± 9 ka and at 257 ± 13 ka. An unusual characteristic is their low ¹⁸O signature. Few data are available on their mineralogy and glass geochemistry, and this study fills an important gap in understanding their petrogenesis. We report new mineralogical observations and plagioclase, whole-rock and glass compositional data. Based on our observations, we describe a new lava flow for which we propose the name East Biscuit Basin flow. This unit is a quartz- and sanidine-free low-silica rhyolite (71–72% SiO₂ in the whole-rock) in which the dominant mineral, plagioclase, comprises two populations: (1) small fresh euhedral crystals of An_20–48 (average of An₃₁) composition, commonly part of aggregates with pyroxenes and Fe–Ti oxides; (2) large sieve-textured isolated crystals, which are slightly more sodic in composition (An_19–34, average of An₂₇). Plagioclase compositions in most other Upper Basin Member rhyolites are similar. The range of compositions for trace elements such as Rb, Th, Y and the rare earth elements is small (e.g. 158–189 ppm Rb, 20–25 ppm Th, 52–63 ppm Y, 60–82 ppm La in the whole-rock), and there is no systematic variation of these elements as a function of SiO₂ content or mineralogy. Certain trace element signatures and ratios are specific to each of these rhyolites, allowing us to propose that the Upper Basin Member rhyolites originate from six independent magma batches. The coexistence of the two types of plagioclase and their progressive disappearance in the more evolved rhyolites suggest the following petrogenetic model for each magma batch. A low-¹⁸O rhyolitic protolith is heated by replenishing magmas, which initiate melting, forming a crystal mush. Replenishment by buoyant silicic magma may enhance melting and cause mixing with the mush material. As a consequence, crystal-poor eruptible magma batches are formed, which contain small, more calcic plagioclase crystals (aggregates) formed during cooling and mixing of the replenishing silicic melt, and larger, lower temperature crystals exhibiting dissolution features inherited from the protolith.

Conditions and Timing of Pumpellyite-Actinolite-facies Metamorphism in the Early Mesozoic Frontal Accretionary Prism of the Madre de Dios Archipelago (Latitude 50{degrees}20'S; Southern Chile)

Willner, A. P., Sepulveda, F. A., Herve, F., Massonne, H.-J., Sudo, M. — Wed, 18 Nov 2009 06:20:40 PST

The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite–actinolite facies: pumpellyite ± actinolite–chlorite ± grandite ± phengite ± epidote–albite–quartz–titanite ± K-feldspar ± calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite–actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K₂O–Na₂O–CaO–FeO–O₂–MgO–Al₂O₃–TiO₂–SiO₂–H₂O–CO₂ at 200–400°C and 1–9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290–310°C, 4–6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 ± 1 kbar, 200–220°C to 5 ± 1 kbar, 290–330°C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. ⁴⁰Ar/³⁹Ar spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233·2 ± 1·8 Ma and in an undeformed metabasite at 200·8 ± 2·4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. ⁴⁰Ar/³⁹Ar isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152·0 ± 2·2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode.

Lithospheric Removal as a Trigger for Flood Basalt Magmatism in the Trans-Mexican Volcanic Belt

Wed, 18 Nov 2009 06:20:41 PST

The voluminous succession of tholeiitic basalts, calc-alkaline andesites and minor high-K basalts that form the Late Miocene Altos de Jalisco mafic province of the western Trans-Mexican Volcanic Belt is interpreted as the magmatic manifestation of a lithospheric dripping event, which removed mantle lithosphere and lower crustal lithologies beneath the study area. During this process, the release of fluids from the foundering materials, coupled with mantle upwelling around the sinking mass, promoted abundant melting of a spinel peridotite and the production of large volumes of tholeiitic magma with low La/Yb and Gd/Yb ratios. Negative correlations of these ratios with MgO contents, Nd isotopes and Rb/Nd ratios indicate that the parental basalts subsequently experienced high-pressure fractional crystallization and contamination with a newly exposed felsic continental crust, thus producing the more evolved calc-alkaline compositions. Stronger garnet signatures and marked enrichments in highly incompatible elements in the high-K suite support derivation from a garnet- and phlogopite-bearing pyroxenitic source, presumably formed by reaction of mantle peridotites with hydrous silicic melts derived from the foundering lithologies. This new petrogenetic model for the Altos de Jalisco volcanic district suggests that the loss of mafic lower crust during lithospheric dripping might be balanced by production of abundant flood basalts within continents, and thus indicates that additional mechanisms may be required for the stabilization of andesitic crust on Earth.