TRACE ELEMENTS IN MAJOR AND ACCESSORY MINERALS AS GUIDES TO UNDERSTANDING THE GEOCHEMICAL AND TECTONOTHERMAL EVOLUTION OF OROGENIC BELTS

A more complete understanding of orogenic processes requires better constraints on the rates and duration of thermal events, where and when partial melts are generated in the continental crust, and how the volume and duration of melts present affect tectonic evolution. However, addressing these issues requires that we accurately interpret the complex histories of rocks in metamorphic belts. Petrographically controlled trace element and isotopic analysis of major and accessory minerals can provide insights into the progressive evolution of rocks through single and multiple events, and allows linking of ages to specific events and physio-chemical processes.

A detailed study of the Brattstrand Bluffs, Prydz Bay, east Antarctica, is demonstrating the utility of these integrated techniques to trace the progressive geochemical evolution of rocks during melting and subsequent tectonic reworking. Moreover, these data are also helping to decipher a complex geologic evolution. Initially thought to have experienced a single cycle of high-T metamorphism between ~540-500 Ma, the recent work has indicated that extensive partial melting and formation of abundant migmatite and leucogranite actually occurred at ≥900 Ma, and that the abundant younger ages are associated with a second event involving extensive reworking and emplacement of granitic magmas.

Analysis of garnet, zircon and monazite in a suite of rocks has helped to characterize the ≥900 Ma melting event. Results show that abundant zircon grew at the source of melting and that mineral entrainment, therefore trace element composition of crustal melts, is likely controlled by the efficiency of melt extraction during migmatization. Enrichment of zircon and monazite in melt residues likely provides a source of radiogenic heat during subsequent reworking events. In addition, trace element zoning in garnet interpreted in the context of compositional variations in zircon and monazite records progressive trace element-depletion of melts during extraction, transport and crystallization. Data suggest that magmas emplaced during the ~540-500 Ma reworking event were likely derived from both fertile and depleted crustal sources, the latter likely reflecting minor re-melting of previously metamorphosed Brattstrand crust.