Journal of Petrology, Volume 40, Issue 4: April 1999.

Inherited Palaeozoic and Mesozoic Rb-Sr Isotopic Signatures in Neogene Calc-alkaline Volcanics, Alborán Volcanic Province, SE Spain^[dagger]

H. P. ZECK¹, A. B. KRISTENSEN² AND E. NAKAMURA³

¹GEOLOGICAL INSTITUTE, COPENHAGEN UNIVERSITY, ØSTERVOLDGADE 10, 1350K COPENHAGEN, DENMARK
²BAKER ATLAS GEOSCIENCE (DK), JORCKS PASSAGE A-4, 1162K COPENHAGEN, DENMARK
³INSTITUTE FOR STUDY OF THE EARTH'S INTERIOR, OKAYAMA UNIVERSITY, MISASA, TOTTORI-KEN 682-01, JAPAN

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[sgr]) 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[sgr]) 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[sgr]) Ga, T_CHUR^Nd crustal derivation ages from ~0·75 to 1·05 Ga and [epsilon]_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.

Keywords: inherited isochrons;crustal anatexis;restite-melt repartition; post-collisional magmatism

Pages 511-524