Temporal, geochemical and isotopic constraints on plume-driven felsic and mafic components in a Mesoproterozoic flood rhyolite province

Abstract

Formation of the Gawler felsic large igneous province (LIP) involved a mantle plume and mafic underplating underneath a metasomatised subcontinental lithospheric mantle (SCLM). Minor extrusive basalt to andesite rocks show the evolution of mantle composition during the initial stages of LIP formation (ca. 15931590 Ma). The earliest basaltic magmatism has low Nb/Yb ratios, low (La/Yb)N, and high ε_Nd(i), representing a relatively depleted region from within the metasomatised SCLM. The more depleted composition and higher volume of this magmatism is consistent with early plume-head driven melts. Subsequent basaltic magmatism shows increasing enrichment in the source, evidenced by increasing Nb/Yb and Th/Nb ratios and decreasing ε_Nd(i). The final stage of basaltic magmatism is marked by an isotopically juvenile SCLM source characterised by lower Th/Nb ratios, possibly representing a rejuvenation phase.

Dacite to rhyolite rocks record the behaviour of crustal melting above a mantle plume. Older dacite to rhyolite units extruded 1593.611588.47 Ma represent compositionally heterogeneous rocks that correlate temporally with the basaltic to andesitic units. The Nd isotopic record suggests an increasing SCLM component from the early to late stages of felsic magmatism, occurring synchronously with increasing enrichment in basaltic magmas. Younger dacite to rhyolite units (1586.661586.39 Ma) have no associated mafic component, are geochemically and isotopically homogeneous and represent large volumes of felsic lava extruded in 300 Kyr. The final stage of felsic magmatism (1586.39 Ma) is marked by crystal-rich dacite, decreasing ε_Nd(i) and lower Ti-in-zircon temperatures which are interpreted to represent the waning phase of the mantle plume and increasing crustal melting.

This study provides a rare example of temporally constrained geochemical and isotopic compositions in mafic and felsic volcanism in a LIP. Our detailed petrogenetic work best demonstrates the generation of the Gawler LIP occurred in an intracontinental, plume setting analogous to a mafic LIP model.