Arc picrite–potassic adakitic–shoshonitic volcanic association of the Neoarchean Sigegudda greenstone terrane, western Dharwar craton: Transition from arc wedge to lithosphere melting

Abstract

Listen

Translate article

A rare association of basalt, andesite, dacite and ultrapotassic volcanic flows is present, together with basalts, komatiites, and banded iron formation (BIF), in the Neoarchean Sigegudda greenstone terrane of the western Dharwar craton. Andesites and dacites are porphyritic, preserving biotite, K-feldspar, and relict fine-grained leucite. Ultrapotassic flows preserve phenocrysts of relict leucite, relict pyroxene, and K-feldspar. Compositionally, basaltic samples are tholeiitic, andesites and dacites are calc-alkaline, and the ultrapotassic samples shoshonites to leucitites. High-Mg basaltic flows have Mg# 0.67–0.52, Ni 1126–260ppm, fractionated REE [(La/Sm)N=1.02–1.75, (Gd/Yb)N=1.41–1.74], and negative anomalies of Nb, Ta, and Ti on primitive mantle normalized diagrams; they are interpreted as arc picrites generated by slab dehydration and high-temperature, low degree, melts of subarc mantle wedge. Andesites qualify as high-Mg andesites. With dacites, these flows have some of the characteristics of adakitic rocks in being aluminous (Al2O3=14–24, 15–18wt.%), with fractionated REE [(La/Sm)N=1.90–2.56, 1.99–2.85, (Gd/Yb)N=1.60–2.64, 1.31–2.50 respectively], with flat to positive anomalies of Zr–Hf relative to MREE, but are potassic relative to “normal” arc andesites–dacites, and possess low abundances of Cr, Co, Ni relative to true adakites. Given the relict mineralogy, with LREE-enrichment the potassic character appears primary. These flows are interpreted as melts of lower arc crust, akin to potassic adakites documented from orogenic belts. Shoshonites and leucitites (SiO2=53–66, 48–53, K2O=3–8, 8–9wt.% respectively) are aluminous at 15–33wt.% Al2O3, with extreme enrichment of LREE at La 200–1000 times chondrite, fractionated REE [La/Sm)N=2.01–3.06, 1.32–3.94, (Gd/Yb)N=2.01–3.65, 1.33–4.50, respectively], and pronounced negative troughs at Nb–Ta–Ti: these compositions are consistent with decompressional melting of arc mantle lithosphere, previously metasomatized by subduction-derived fluids, during arc rifting. Accordingly, this volcanic association records a transition from subarc wedge melting to melts of arc crust and mantle lithosphere, likely in response to arc rifting.