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Título

HISTORY OF POLYBARIC CRYSTALLISATION IN SHOSHONITIC AND LATITIC MAFIC MAGMAS IN THE RIO DO CACHORRO STOCK, SERGIPE

Texto do resumo

The Rio do Cachorro Stock (1 km2) is a monzonitic intrusion in the Macururé Domain's central portion of the Sergipano Orogenic System. It is one of several shoshonitic intrusions in the Macururé Domain and has a U-PbShirimp crystallization age of 621 ± 5 Ma. The presence of enclaves is a striking feature of this pluton, with many microgranular (EMM) and cumulate mafic enclaves. These microgranular mafic enclaves occur either dispersed or grouped in synplutonic dykes or enclave corridors. They are composed of micro-monzonites and micro-monzodiorites, which chemically correspond to shoshonites and latites. The orthoclase-cumulate correspond to alkali-feldspar-sienites. The mineralogy of these rocks is plagioclase, alkali feldspar, biotite, clinopyroxene, amphibole, quartz, titanite, apatite, epidote, allanite, zircon, and opaque minerals. The chemical data of the minerals in the microgranular mafic enclaves shows that alkali feldspar is albite (An<5) and orthoclase; plagioclase corresponds to albite (An5-10) and oligoclase (An11-28); brown mica is biotite and phlogopite; clinopyroxene is composed of diopside and augite; amphibole is magmatic, calcareous and composed of Mg-hornblende, Mg-hastingsite, edenite, pargasite and Mg-sadanagaite; and actinolite is late magmatic. Thermobarometer calculations made with mineral chemistry data showed a long polybaric evolutionary history in these enclaves: clinopyroxene crystallized along the rise of the magmas from depths of 62 to 26 km (17-7 kbar) at temperatures of 1217-985 ºC; amphibole crystallized from depths of 30 to 14 km (8-4kbar) at temperatures of 939-801 ºC, respectively; and titanite is magmatic and crystallized at temperatures of 1017 ºC to 855 ºC. The enclaves are saturated in silica (SiO2= 49.95% to 54.33%), metalluminous, alkaline, have 5<(Na2O+K2O)<9, 1<(K2O/Na2O)<3, are enriched in LILE, exhibit ETRLeves fractionation and correspond to shoshonitic and latitic magmas. The compositional variations identified, and the evolution of trace elements suggest a mantle source with fractionation of amphibole clinopyroxene, feldspar, and accessory minerals to form the intermediate mafic magmas corresponding to the enclaves.

Palavras Chave

Mineralochemistry; Thermobarometry; shoshonite magmas