Characterization of the Mefjell plutonic complex from the Sor Rondane Mountains, East Antarctica: Implications for the petrogenesis of Pan-African plutonic rocks of East Gondwanaland

摘要

The Mefjell plutonic complex consists of 500-550-Ma Pan-African plutonic rocks, which intrude into the Precambrian crystalline basement in the Sor Rondane Mountains, East Antarctica, and forms part of the Sor Rondane Suture Zone. The complex comprises syenitic and granitic (mostly monzogranitic) rocks, and is characterized by the presence of iron-rich hydrous mafic minerals and primary ilmenite, both of which imply its formation at high temperature and under low oxygen fugacity conditions. The syenitic rocks are metaluminous, and are high in alkalis, K2O/Na2O, Al2O3, FeOt/(FeOt + MgO) (0.88-0.98), K/Rb (800-1000), Ga (18-28 p.p.m.), Zr (up to 2100 p.p.m.) and Ba. They also have a low Mgsharp (Mg/[Mg + Fe2+]), Rb, Sr, Nb, Y and F, low to moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios and positive Eu anomalies in their rare earth element (REE) patterns. The granitic rocks are metaluminous to peraluminous, and have a high Rb content, high Sr/Ba and LREE/HREE ratios, low K/Rb and negative Eu anomalies. Most of the syenitic and granitic rocks have Y/Nb ratios greater than 1.2, and are depleted in Nb, Ti and Sr on the primitive mantle-normalized spider diagrams, indicating a crustal origin with subduction zone signatures. We interpret both the syenitic and granitic rocks to be derived from an iron-rich lower crustal source by dehydration melting induced by the heat of mantle-derived basaltic intrusion, after which they then underwent limited fractional crystallization. The Mefjell plutonic complex has a high Zr content and tectonic discrimination diagram signatures indicative of normal A-type granitic rocks. Both rock suites may have been generated under the same postorogenic tectonic setting. The Mefjell syenitic rocks are chemically comparable to charnockites in the Gjelsvikjella and western Muhlig-Hofmannfjella areas of East Antarctica, whereas the granitic rocks are comparable to aluminous A-type granitic rocks in South India, which were emplaced during formation and evolution of the Gondwanaland supercontinent.