The A-type Pirrit Hills Granite, West Antarctica: an example of magmatism associated with the Mesozoic break-up of the Gondwana supercontinent

Abstract

The Mesozoic geology of West Antarctica is largely related with the break-up of the Gondwana supercontinent and offers a good example for understanding magmatism associated with the continental break-up process. West Antarctica can be divided into five crustal blocks with relatively thin crust. The blocks are separated by deep rift zones and have moved during the Mesozoic break-up of Gondwana. The Pirrit Hills granite occurs as an isolated pluton in the Ellsworth-Whitmore Mountains block, which is the center of five blocks in the present configuration. The granite consists of quartz, perthitic alkali feldspar, and plagioclase with minor amounts of interstitial biotite and muscovite. The granite is a highly homogeneous, strongly fractionated, and mildly peraluminous granite and belongs to A-type granites with A2-type characteristics, suggesting its generation in an anorogenic environment. The strong enrichment of HREE and significant negative Eu anomalies suggest that the granitic magma was produced by a small degree of partial melting of a garnet granulitic source in the unusually hot lower crust. A weighted mean 206Pb/238U age of zircons is 164.5 ± 2.3 Ma (MSWD = 1.3), which is 8 to 9 Mys younger than a former Rb-Sr whole rock age (173 ± 3 Ma), and corresponds to the first rifting stage of the break-up of Gondwana (at 165 Ma). We suggest this age to be the emplacement age of the Pirrit Hills granite. The A-type Pirrit Hills granite was emplaced in the Middle Jurassic accompanying crustal thinning due to the break-up of Gondwana.