Composições químicas de biotita como discriminador petrogenético: Batólito Rio Jacaré, Província Borborema, NE do Brasil

Chemical compositions of biotite as a petrogenetic discriminator: Rio Jacaré Batholith, Borborema Province, NE Brazil

Abstract

Biotite is the most common mica in igneous rocks, but until the beginning of this century, researchers used amphibole and pyroxene more frequently to estimate the conditions of magma crystallization. Currently, biotite has become a useful petrogenetic tool and its primary chemical composition can also be used to infer magmatic crystallization conditions. Primary and reequilibrated biotite are found in the Rio Jacaré Batholith (RJB) rocks. Primary crystals mainly occur as inclusions in plagioclase and have a composition of crystals formed in calc-alkaline orogenic magmas. The crystallization temperature of biotite primary of the RJB ranged from 678 to 745ºC. Pressure during magmatic crystallization of biotite ranges from 1.2 to 2.9 kbar. This is consistent with the crystallization temperature and pressure of biotite in granitic systems. The compositions of the primary crystals indicate that they were formed from magmas with H₂O contents between 5 and 7%. The fO₂ during the formation of these crystals range from -16.3 to -13.9. When correlating temperature variation and fO₂ variation of the primary crystals in the different enclaves samples, it is possible to infer the presence of several mafic magmatic pulses during the evolution of the RJB.