GEOCHEMICAL AND SR-ND ISOTOPIC MAPPING OF SOURCE PROVINCES FOR THE MAWSON CHARNOCKITES, EAST ANTARCTICA - IMPLICATIONS FOR PROTEROZOIC TECTONICS AND GONDWANA RECONSTRUCTION

Large charnockite plutons (similar to 970 Ma) were intruded into high- grade metamorphic rocks of the Mawson Coast area, east Antarctica, imm ediately after peak granulite metamorphism in the region. Detailed Sr- Nd isotopic studies suggest that charnockites in the central zone have significantly higher initial Sr-87/Sr-86 ratios (0.7205-0.7334), lowe r initial epsilon(Nd) values (-8.8 to -11.1) and older Nd depleted man tle model ages (T-DM(Nd)) (2.03-2.16 Ga) than those from the western a nd eastern zones (0.7076-0.7263; -4.0 to -8.4; 1.68-1.96 Ga, respectiv ely). The isotopic compositions of the country rocks also mimic such r egional variations. Apart from the isotopic variations, the charnockit es can also be divided into two major geochemically distinctive catego ries, i.e. high-and low-Ti groups, with-the high-Ti group being charac terized by higher TiO2, P2O5, K2O, FeO, Na2O + K2O, Zr, Nb, Y, La, Ce , Rb, Pa, Rb/Sr and Ga-Al and lower MgO, CaO, Al2O3, Na2O, Mg#, Cr and Ni than the low-Ti group. However, the two groups show no obvious dif ference in terms of Sr-Nd isotopic signatures. Compared with Palaeozoi c I-type granites from the Lachlan Foldbelt, both groups of charnockit es are generally higher in TiO2, P2O5, K2O, Zr, Nb, Y, La, Ce and Ba, and lower in MgO, CaO, Sr/Ba and Mg#, with the low-Ti charnockites bei ng somewhat transitional between the high-Ti charnockites and I-type g ranites. On the spiderdiagram, both charnockites and I-type granites s how similar degrees of negative Nb anomaly, implying derivation from s imilar Nb-depleted sources. We consider the Mawson charnockites, like those in the northern Prince Charles Mountains (PCM), were derived by high-temperature partial melting of dry, granulitic crustal sources, w ith charnockites in the central zone being related to a significantly older source province than those in the western and eastern zones. The charnockite magmatism occurred probably in response to Meso-Neoproter ozoic continental collision related to construction of the Rodinia sup ercontinent. The igneous charnockites in the Mawson-PCM areas and the provincial distribution of their T-DM(Nd) ages and initial Sr-87/Sr-86 ratios are potentially important markers for precise reconstruction o f the Rodinia and Gondwana supercontinents. This study demonstrates th at the isotopic systematics, especially the T-DM(Nd), ages in granitic rocks, may provide an important means for the delineation of distinct ive crustal terranes and their tectonic configuration, especially in s tructurally complex regions. (C) 1997 Elsevier Science B.V.