Pyroxene and amphibole chemistry of peralkalic ultra-potassic syenite and associated pyroxenite, northeastern Brazil: an approach to test liquid immiscibility process

Abstract

Liquid immiscibility has been a process little explored as a mechanism for generation of magmas of contrasting composition, although its importance has been recognized by some petrologists. In this work, the chemistry of mafic phases in ultrapotassic syenite and pyroxenite, which seem to have coexisted as liquids for a certain period of time, in the Late Precambrian Triunfo pluton, Pernambuco, is discussed. The two rock types have the same mineral phases (clinopyroxene, microcline, apatite, sphene, blue amphibole) only at different amounts. The chemistry of pyroxene (aegirine-augite), which is only slightly zoned, and amphibole (K-rich richterite) of both rock types, show that the mineral phases in the pyroxenite are similar to their corresponding ones in the syenite, compositionally overlapping. These features strongly suggest chemical equilibrium between syenite and pyroxenite, which is one of the requirements for two liquids constitute an immiscible pair. This hypothesis is supported by the presence of mixed-rock inclusions that consist of cm-sized, near-rounded, syenitic and pyroxenitic drops mutually interstitial, in an emulsion-like texture, which record magma splitting.