Journal of Petrology | Volume 40 | Number 4 | Pages 511-524 | 1999
© Oxford University Press 1999
Inherited Palaeozoic and Mesozoic Rb–Sr Isotopic Signatures in Neogene Calc-alkaline Volcanics, Alborán Volcanic Province, SE Spain
1 Geological Institute, Copenhagen University Østervoldgade 10, 1350K Copenhagen, Denmark
2 Baker Atlas Geoscience (Dk) Jorcks Passage A-4, 1162K Copenhagen, Denmark
3 Institute for Study of the Earth's Interior, Okayama University Misasa, Tottori-Ken 682-01, Japan
Received November 9, 1997; Revised typescript accepted August 18, 1998
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Abstract |
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A pyroxene andesite unit within the post-Alpine Alborán volcanic province has a Neogene extrusion age; however, its Rb–Sr isotopic relations define a regression line of 509 ± 62(2
) Ma (Early Palaeozoic). There are two concordant data point clusters on the regression line, one of which is well constrained, defining a secondary regression line of 202 ± 30(2) Ma (Early Mesozoic). Considering the mineralogy of the andesites—plagioclase, Ca-poor and Ca-rich pyroxene, and Ti-magnetite—and the presence of restitic aggregates comprising these same four minerals, recent dehydration melting experiments suggest an origin by anatexis of an amphibolite-dominated source rock complex. Inherited zircon ion-microprobe ages in the range of 500–1800 Ma, an Sm–Nd isochron age of 1.5 ± 0.4(2) Ga, TCHURNd crustal derivation ages from 
0.75 to 1.05 Ga and 
Nd(0) values of –4 to –7 support a complex petrogenesis, involving large-scale reworking of older material. 87Sr/86Sr vs 1/Sr and 143Nd/144Nd vs 1/Nd indicate a heterogeneous source rock complex showing two-component mixing. The data favour volcano-sedimentary source rock complex parent material which at 500 Ma underwent a diagenetic or hydrothermal event, which regionally reset Rb–Sr isotope systematics. Subsequently, at 200 Ma the complex went through local diagenetic or hydrothermal re-equilibration, which created domains with slightly different 87Sr/86Sr ratios, before undergoing Alpine high-grade metamorphism and subsequent anatexis. Roughly coeval, restite-rich cordierite dacites show similar, 200 Ma, high-age Rb–Sr isotopic relations, which are interpreted as the age of diagenesis of its sedimentary parent material. This is supported by inherited zircon ion-microprobe ages of 300–400 Ma upwards. Also for these rocks 87Sr/86Sr vs 1/Sr shows linear trends, which are explained analogically by sedimentary component mixing in the parent material of the anatectic source rock complex rather than by magmatic stage mixing or contamination. A sinking slab model is suggested for the regional setting of the crustal anatectic regime, melting being supported by fast uplift (of isotherms) and diapiric underplating by high-temperature asthenospheric mantle.
KEY WORDS: inherited isochrons; crustal anatexis; restite–melt repartition; post-collisional magmatism
* Corresponding author. Present address: Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori-Ken 682-01, Japan Telephone: 81 858 43 3876. Fax: 81 858 43 2184. email: zeck{at}misasa.okayama-u.ac.jp
Dedicated to Professor Ikuo Kushiro on the occasion of his retirement 31 March 1999 as director of the Institute for Study of the Earth's Interior, Okayama University at Misasa.