Multi-stage interactions between mantle derived magmas and crustal melts: an example of the Devonian magmatism of Sierra de San Luis, Argentina. Symposium 3 Contaminated granites

LÓPEZ DE LUCHI, M. G., ; SIEGESMUND ,S; STEENKEN, A; WEMMER, K; NAUMANN, R

Dalhousie University,Halifax

Otro; GAC-MAC 2005, Halifax 2005; 2005

Geological Association of Canada-Mineralogical Association of Canada

Devonian alcali-calcic granitic intrusions of the Sierra de San Luis consist of granodiorites, monzogranites and leuco-monzogranites associated with a Mg-K series of vaugnerite, hornblende?biotite diorite, monzonites and quartz-monzonites that appear as intrusive stocks, synplutonic dykes and enclaves. Metamorphic enclaves in the high-K granites are metasediments of pelitic?psammitic composition, and some disrupted melanosomes rich in garnet and symplectitic biotite that are restites from partial melting. Syn-plutonic dykes, stocks and most of the microgranular enclaves carry xenocrysts derived from the host magma, and some contain cores of a high-Mg pyroxene derived from a mafic magma, in the amphibole phenocryst. Alcali-calcic granitoids are characterized by high LILE, TiO2, P2O5, Ga, Zr and Nb, low CaO, Yb (c.a 2), Sc ( 300ppm) with (La/Yb)N (50-20). LREE are highly fractionated, (La/Sm)N is 10 and HREE are less fractionated with (Tb/Yb)N around 4 which indicate amphibole fractionation. The monzonitic rocks show very high concentrations in LILE, Sr, Mg, Cr (up to 250 ppm), Ni, V, LREE, Zr and Th. High Sr/Y, low Yb, strongly fractionated REE pattern and a smooth Eu anomaly indicate garnet as a major residual phase with minor plagioclase. Large scale melting of kilometre sized regions of the crust is required to generate large granitic intrusions like the voluminous Devonian magmatism. Vaugnerite, only present as enclaves in the stocks, originated from amphibolitized lamprophyric magmas probably derived from enriched mantle melts that were injected into the lower crust and provided the thermal input for the extensive partial melting that originated the high-K granitic melts. Deep-seated interactions between felsic and mafic magmas were responsible for the characteristics of both magma suites. The alkali-calcic and metaluminous character of the Devonian granitoids may correspond to the anatexis of an hornblende rich source. Stocks and syn-plutonic dykes represent hybrid mafic magma commingled/contaminated by felsic host magma whereas microgranular enclaves formed either as globules of the hybrid mafic magma or as disrupted fragments of the stocks and dykes. Crystal fractionation of hybrid magmas together with in situ mingling are the two processes that control the chemical evolution of the Mg-K series, Epsilon Nd from ?1.4 to ?2.8 and 87Sr/86Sr from 0·7038 to 0·7043 in the quartz monzonites reflect an increasing contribution of crustal components in the more differentiated rocks of the Mg-K series.