PERALUMINOUS PLUTONISM: NATURE AND ORIGIN OF THE MOLY MAY LEUCOGRANITE AND ITS COAST PLUTONIC COMPLEX GRANITIC HOST-ROCKS, NORTHWESTERN BRITISH COLUMBIA

Abstract

The Moly May pluton of northwestern British Columbia is one of several small molybdenite-bearing granitic complexes of Cenozoic age known as the Alice Arm intrusions. These were intruded into the eastern margin of the Coast Plutonic Complex (CPC) and the Bowser Lake Group (greenschist-grade metasedimentary rocks). This pluton consists of three mappable units (aplitic garnet–muscovite leucogranite, medium-grained leucogranite, and coarse-grained to pegmatitic biotite granite), in addition to a molybdenite-mineralized leucogranitic variety. The Moly May leucogranites (MML) and the CPC granite–granodiorite host rocks are chemically distinguished by strikingly different geochemical trends, which indicate that the two granitic suites are not genetically related. The MML is exclusively peraluminous (molar A/CNK in the range 1.00 to 1.78), whereas the CPC rocks are metaluminous to weakly peraluminous (molar A/CNK in the range 0.84 to 1.06). The MML is significantly enriched in K, Rb, Nb, Y, and depleted in Mg, Ca, Na, Ba, Sr, and Zr compared to the CPC. The REE patterns of the MML are fractionated [(La/Yb) N = 23] and show a minor negative Eu anomaly, but those of the garnet-bearing leucogranitic variety lack the REE fractionation [(La/Yb) N = 2], and have a significant negative Eu anomaly. The mineralized leucogranite is strongly depleted in the LREE , K, Rb, and Nb; these elements are interpreted to have been leached out at subsolidus temperatures by mineralizing hydrothermal solutions, which led to deposition of molybdenite. Compared to the REE patterns of the MML, those of the CPC are slightly more fractionated [(La/Yb) N = 27], and lack a Eu anomaly. The MML was developed in association with an Eocene transtensional tectonic regime. Chemical features suggest that the MML is of a sedimentary origin. Results of geochemical modeling show that the leucogranitic magma was produced by a large degree (65%) of batch partial melting of a metasedimentary protolith.