Cryptoperthitic and replacive intergrowths with iridescence in monzonitic rocks from Cerro Colorado, northern Chile

Abstract

Cryptoperthitic alkali feldspars in the Cerro Colorado monzonites from the Atacama Desert, northern Chile, exhibit bluish iridescence. Three types of cryptoperthites are developed over a grain; fine and coarse cryptoperthites of Or-Ab-An ternary compositions around Or 60–65 Ab 35–30 An 5 , and Or-rich, An-poor cryptoperthite around Or 70 Ab 26 An 4 . The former ternary compositions point to crystallization at approximately 900°C. Hydrothermally turbid patch microperthites are not developed in the alkali feldspar, whereas turbid Or-rich veins with micropores display limited replacement of the cryptoperthites along the lamellae. The Or-rich veins are much richer in water than the cryptoperthitic regions. Blue cathodoluminescence of variable tint is observed over individual grains, except in the Or-rich veins. The Cerro Colorado alkali feldspars record an incipient state of alkali feldspars, which predates the low-temperature formation of patch microperthite in subsolvus granites. These feldspars are the first natural alkali feldspars with K-feldspathization without microperthite formation.