Syn-volcanic cannibalisation of juvenile felsic crust: Superimposed giant 18O-depleted rhyolite systems in the hot and thinned crust of Mesoproterozoic central Australia

Low‐δ18O Rhyolites From the Malani Igneous Suite: A Positive Test for South China and NW India Linkage in Rodinia

“…None of the xenocrystic zircon grains have low‐δ 18 O values, indicating that the heat source for the hydrothermal alteration also produced the MIS low‐δ 18 O rhyolites. This requires high‐temperature remelting of crustal materials in a shallow (upper crustal) magma chamber, where hydrothermal alteration of an isotopically heavy δ 18 O precursor by a low‐δ 18 O meteoric water occurred prior to or nearly simultaneously with eruption (Bindeman & Valley, ; Smithies et al, ). Given the highly radiogenic zircon Hf isotopic compositions of the low‐δ 18 O rhyolites (Figure ), the primary upper crustal source involved in the bulk cannibalization was depleted in nature.…”

“…Physical evidence for early MIS phase with mantle‐like Hf‐O isotopic composition prior to hydrothermal alteration by low‐δ 18 O meteoric water is supported by the xenocrystic zircon from the Punagarh dacites with mantle like Hf‐O isotopic signatures (δ 18 O zircon = 5.62‰ and Ɛ Hf( t ) = +6.6) dated at 795.0 ± 11.4 Ma (Figure ). Nonetheless, such zircon in this potential mafic precursor was rarely preserved in the subsequent zircon‐rich low‐δ 18 O rhyolite, because zircon is generally undersaturated in basaltic magmas and the subsequent remelting occurred at relatively higher temperatures (e.g., Smithies et al, ). Comagmatic mafic rocks, however, do exist at ~60–100 m below the surface, also documented in the borehole data in the Jodhpur area (Pareek, ).…”

“…The rare coincidence of sustained high temperatures at upper crustal levels with hydrothermally altered crust limits the distribution of voluminous depleted δ 18 O felsic volcanic system (Boroughs et al, ; Smithies et al, ). Bulk cannibalization of juvenile mafic crust to produce low‐δ 18 O rhyolites requires remelting of shallow hydrothermally altered volcanic rocks that had been tectonically (rift zones) and/or volcanically (calderas) buried (Bindeman et al, ; Drew et al, ; Watts et al, ).…”

“…Magmatic rocks with zircon δ 18 O lower than the mantle value (+5.3‰; Valley et al, 1998) are rare as they require large quantities of meteoric water for high-temperature exchange (Smithies et al, 2015;Valley et al, 2005). The maximum depth, at which water circulation and the hydrothermal O-isotopic exchange can occur effectively, is within the upper~10 km of crust, limited by the brittle-ductile transition (Menzies et al, 2014).…”

“…The maximum depth, at which water circulation and the hydrothermal O-isotopic exchange can occur effectively, is within the upper~10 km of crust, limited by the brittle-ductile transition (Menzies et al, 2014). Therefore, the low-δ 18 O silicic volcanic rocks can provide precise information on magmatism and uppermost crustal processes (Bindeman et al, 2012;Smithies et al, 2015). Besides, low-δ 18 O silicic magmas are also significant for understanding caldera and/or rift tectonic settings on account of their close association such as the Holocene Iceland magmatism, which was considered to be generated by partial melting of crust that is hydrothermally altered in shallow (rift) environments and with diverse δ 18 O values (Bindeman et al, 2012 (Archibald et al, 2016;Chen et al, 2011;Fu et al, 2013;Liu & Zhang, 2013;Zheng et al, 2007).…”