REINTERPRETAÇÃO DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS, DE ACORDO COM OS SISTEMAS VULCANO-PLUTÔNICOS DE SUBSIDÊNSIA. PARTE 2: QUÍMICA MINERAL, GEOQUÍMICA E ISÓTOPOS DE PB-SR-ND

Abstract

The post-collisional Lavras do Sul intrusive complex- LSIC, southern Brazil, is associated with basic-to-intermediate rocks derived from two lithospheric mantle segments. All rocks show a geochemical subduction signature, with negative Nb anomalies. Minettes occurring as dykes are potassic, alkalic rocks containing olivine (F 091-86 ), Cr-diopsideand Ba-rich Ti-phlogopite, whose chemistry is only slightly modified by crustal processes. They are enriched in LREE, LILE and HFSE, and have high radiogenic isotopic ratios, so they derived from an enriched mantle previously modified in subduction events. We conclude that they register the change to an extensional regime that had occurred during the later magmatic LSIC-events. The others rocks are more evolved and have a shoshonitic composition, including trachyandesites, spessartites and the Arroio do Jacques monzodiorite (AJM). They have augite as the main mafic phase, and variable trace element sand Pb-Sr-Nd isotopic ratios. They derived from the mantle wedge produced during Neoproterozoic orogenies, but with a more prominent role of open-system differentiation processes. The interaction with minette magma is proposed for AJM that has intermediate trace element content sand isotopic ratios. The two groups of LSIC-granitoids were similarly originated from two parental magmas. Most have a shoshonitic affiliation, including the Tapera monzonite-TM in the north, and the core facies of the southern body (granodiorite and hybrid granites). Alkaline granites (syenogranite and perthite granite) form semicircular stocks around this core. Contrasts in the emplacement depth of these rocks, revealed by Al t -in amphibole geobarometer, imply in more than one cycle of subsidence-resurgence. Granites from the core, classified as low temperature l-type granites, had a major crustal contribution. They formed during the resurgence (4-2 kbar) at ca. 600 Ma. Alkaline granites were emplaced at shallow levels (minor or equal to 2 kbar) through successive episodes, between 598 and 586 Ma, that apparently affected the border of those from the core. They involved the interaction of two parental magmas, but with increasing contribution of the alkalic type.