Fertile granites of Precambrian rare-element pegmatite fields: is geochemistry controlled by tectonic setting or source lithologies?

Abstract

Abstract A total of twenty fertile granites and their rare-element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature, namely Li, Rb, Cs, Be, Ga, Sn, Nb Ta, Y, REE, Sc, Ti, Zr, Be, Th, U, F (NYF). The range of ΔT (time difference) between metamorphism and/or deformation and granite intrusion is −20 to 465 Ma for the examined cases. There is some tendency toward syn- to late-orogenic origin of LCT's as opposed to post- to anorogenic timing of NYF's, but the correlation is poor compared to the previously accepted stereotype. Similarly, both metaluminous and peraluminous fertile granites are about equally spread among syn-, late- and anorogenic environments. LCT systems are largely associated with peraluminous granites, whereas all metaluminous to peralkaline plutons bear the NYF signature. Tectonic control on geochemistry of granite-pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts. The LCT-type suites are derived from undepleted upper-crustal lithologies suffering their first anatectic event, which mobilizes the most volatile components into low-temperature, low-percentage melts. Supracrustal sequences as well as ortho- and para-lithologies of their basement serve as protoliths, with variable degrees of involvement. The NYF-type suites whose plutons have the geochemical signature of A-granites, may be derived by melting of depleted lower-crustal sources, residual after preceding anatectic events, or possibly by melting of juvenile lithologies with short crustal residence. Differentiation from mantle-derived magmas is highly improbable. Volatile extraction of NYF elements from the mantle and their pre-enrichment in protoliths or melts seems to be required. The few mixed systems with variable expressions of a NYF⪢LCT signature are derived basically as NYF suites but contaminated by the LCT elements from undepleted upper crust via several possible mechanisms.