Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

摘要

The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the orgin and evolution of the magmas postdating the Ross Orogen. The delta~(18)O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts (delta~(18)O_(WR)=6.8-7.4 per thousand). The O-isotope character of the mantle source inferred from the delta~(18)O values of cumulous olivine in gabbros (5.7-6.8 per thousand) is enriched in ~(18)O compared to modern arc-related magmas. Geochemical data and concurrent high delta~(18)O values, and initial strontium (~(87)Sr/~(86)Sr=0.7060) and neodymium (~(143)Nd/~(144)Nd=0.5122) isotope ratio indicate that the olivine gabbros formed by crustal contamination of a primary calcalkaline basaltic melt. The diorites have high delta~(18)O values, among the highest ever measured for dioritic rocks (8.7-10.3 per thousand), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708-0.710). The diorites have pyroxene with high, nearly constant detal~(18)O values (8.2-8.6 per thousand) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calcalkaline basalt experiencing different amounts of crustal contamination, and underwent different degress of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The delta~(18)O, ~(87)Sr/~(86)Sr and ~(143)Nd/~(144)Nd compositions of the granites and granodiorites overlap (10.8-12.1 per thousand, 0.7096-0.7108, 0.5119-0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile metal-igneous protoliths.