The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks |
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| Authors: | HD Mensel MT McCulloch BW Chappell |
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| Institution: | 1. Department of Geology, Australian National University Canberra, A.C.T.Australia;2. Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia |
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| Abstract: | Nd and Sr isotopic compositions are reported for the granitic suites which comprise the late Palaeozoic to earliest Mesozoic New England Batholith of eastern Australia. Some of the granitic suites are typically I-type in their mineralogy, chemistry and isotopic compositions, implying a derivation from igneous (infracrustal) source rocks, whereas other suites have characteristics consistent with a derivation from a protolith which was predominantly sedimentary and relatively felsic (S-types). The I-type granitoids of the Nundle Suite have ?Nd values (+3.3 to +6.1) that are amongst the most primitive yet documented for a relatively felsic (SiO2 ~ 65%) plutonic suite and these values imply a derivation from either a depleted upper mantle source or, more probably, a complex source region involving both volcanic-arc rocks and detrital material. Their compositions are distinctly more primitive than those of the New England Super-Suite which constitutes the Permian ‘core’ of the batholith. This extensive Super-Suite (comprising granitoids traditionally designated as I-type) has an overall range in initial Nd and Sr isotopic compositions of ?1.7 to +4.6 and 0.70458 to 0.70624 respectively, although the majority of plutons have initial Nd isotopic compositions which fall into a very narrow range (+1.0 ±1.5 ?units). This limited range is remarkable considering the extreme lithological diversity and range in chemical composition of the analysed samples (SiO2 47%–74%) and implies a source region of considerable volume having reasonably uniform isotopic compositions but variation in chemistry. A similarly uniform source isotopically is also indicated for the S-type granitoids of the Carboniferous Hillgrove Suite and Carboniferous-Permian Bundarra Suite with initial ?Nd values of +0.8 to +2.3 and initial compositions of 0.70474 to 0.70577 showing only limited ranges.Five pelites, three ‘felsic’ and four ‘mafic’ greywackes, representing typical country rocks from different stratigraphic levels have initial ?Nd values (?1.7 to +6.6) and initial compositions (0.70378 to 0.70585) which essentially mirror the compositional variation in the granitoids. A chemical, mineralogical and isotopic bimodality in these sediments indicates two very distinctive sources, one felsic (rhyodacitic) the other relatively mafic (andesitic), which were separated spatially and temporally in many but not all areas of early- to mid-Palaeozoic New England. A model is presented in which the S-type granitoids are derived from a predominantly felsic source, i.e. pelites and ‘felsic’ greywackes, whereas some of the granitoids belonging to the New England Super-Suite may have been derived from source rocks consisting of both felsic and mafic sedimentary components.The distinction between many S-type and I-type granitoids in New England is unclear for two principal reasons; (a) because the granitoids and their respective source rocks are relatively young geologically so that their isotopic systems have not evolved to any considerable extent, and (b) because of the intrinsic igneous chemical compositions of any sedimentary component that may be involved in their genesis. |
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