Crustal and Enriched Mantle Derived Melts in the Devonian Magmatism of the Sierra de San Luis, Argentina
Abstract
Devonian granitoids of the Sierra de San Luis consist of voluminous batholiths that were broadly synchronous with the collision between the Chilenia Terrane and southwestern Gondwana and the development/reactivation of NE-trending ductile shear zones. Batholiths were emplaced at around 3.5 kbars. and are made up by high K calc-alkaline porphyric granodiorites and equigranular monzogranites. Granitoids are associated with high Mg-K quartz syenites, monzonites, vaugnerites and durbachites that appears as stocks, syn-plutonic dykes and enclaves in the porphyric facies and shows textural and chemical evidences of mingling and mixing processes. Granitoids are contemporaneous to ultrapotassic lamprophyre dykes (minettes). that intruded the country rocks next to the contacts and preferentially emplaced in an E-W fracture system. The less evolved high Mg-K series rocks present strong enrichment in K, Rb, Ba, Th, Zr anf Hf, high Mg, Ni, Cr, V and relatively low Ti and Nb contents which indicate a basic primitive magma enriched source. Sucessives stages of mixing and mingling with the evolving calc-alkaline melt are indicated by field and petrographic data in synplutonic dykes and enclaves: identical feldspar megacrysts, desaggregation of enclaves that appear as mafic rafts in the porphyroid facies, quartz ocelli with reaction rims, the general predominance of amphibole and euhedral sphene versus biotite in areas where the synplutonic dykes are intruded, transitional contact between hybridized and "normal porphyroid facies, etc. These processes took place mainly prior to emplacement and continue during the entire emplacement as shown by the parallelism between internal and external fabrics. The mantle component of the less evolved hybrid granitoids is indicated by their initial 87Sr/86Sr of 0.70380 and epsilon Nd of -1.4. Therefore high K, Ba and Sr and high field strength element contents of these rocks must be considered as a primary feature. The more evolved granitoids with initial 87Sr/86Sr of 0.70703 and epsilon Nd=-3.1 show values that are lower than expected for a continental source but higher than those of a mantle source. The lamprophyric magma have higher mg number. Correlation between K, Cr, Ba, Mg, REE indicate a source region in a subduction-enriched lithospheric mantle. Lamprophyres represent melts in equilibrium with a metazomatized mantle containing phlogopite and garnet (La/YbN =40). As the minettes display a clear subduction zone signature, the enrichment was not caused by mafic silicate melts, but by K and H-rich fluids that had an ultimate source in crustal rocks. The incompatible element signatures of the high Mg-K series and minettes are strikingly similar with decoupling of Th and Nb from Zr, Hf, and Ti that rely on the relative contents of these elements in constituent minerals and mineral proportions. A model is proposed where the lamprophyres and the high Mg-K series could share a mixed crust and upper mantle source. Crustal thickening and subsequent melting provided purely crustal magmas that mixed with mantle derived magmas and generated the entire spectrum of hybrid high Mg-K series. Variable degrees of partial melting during a period of adiabatic decompression may account for the differences between lamprophyres and high Mg-K series.The generation and emplacement of minette melts may have been promoted by extensional tectonic related to a combination of gravitational collapse of thickened crust, slab break-off, and strike-slip faulting.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V43B..01L
- Keywords:
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- 3600 MINERALOGY AND PETROLOGY (replaces;
- 3640 Igneous petrology;
- 3670 Minor and trace element composition