Abstract
In the Lachlan Orogen of south-eastern Australia, the high-level, postorogenic, 368-Ma, I-type Tynong pluton contains granitic to granodioritic rocks that crystallised from a variety of mainly crustally derived magmas emplaced in the shallow crust, in an extensional regime. The isotopic characteristics of the main plutonic rocks are relatively unevolved (87Sr/86Sr t ~ 0.705–0.706 and εNd t ~ −0.4 to 0.6), suggesting source rocks not long separated from the mantle. We infer that arc mafic to intermediate rocks and associated immature greywackes formed the main crustal source rocks and that these are located in the largely unexposed Neoproterozoic–Cambrian Selwyn Block that forms the basement. As exposed near its southern margin, the pluton also contains minor, pillowed sheet-like intrusions of quartz dioritic rock that show mainly mingling structures with the enclosing granodiorites, as well as some hybrid pods and fairly abundant igneous microgranular enclaves that we infer to have been derived from the quartz dioritic sheets. Despite this evidence of direct mantle input into the Tynong magma system, the main granodioritic series do not appear to have been formed by magma mixing processes. Of any I-type granite in the region, the Tynong pluton has perhaps the most direct connection with mantle magmas. Nevertheless, the main mantle connection here is probably in the mantle-derived protolith for these crustal magmas and in the mantle thermal event that gave rise to melting of the deep crust in the Selwyn Block. This degree of mantle connectedness seems typical for I-type granitic rocks worldwide.
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Acknowledgments
JC acknowledges the South African National Research Foundation for providing support for travel and field and analytical work through its programme of Incentive Funds for Rated Researchers. We thank Tom Sisson and Calvin Barnes who both provided very useful reviews, which assisted us in strengthening our arguments on the efficacy of various mooted petrogenetic processes and in clarifying and extending some of our explanations.
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Communicated by Hans Keppler.
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Clemens, J.D., Regmi, K., Nicholls, I.A. et al. The Tynong pluton, its mafic synplutonic sheets and igneous microgranular enclaves: the nature of the mantle connection in I-type granitic magmas. Contrib Mineral Petrol 171, 35 (2016). https://doi.org/10.1007/s00410-016-1251-y
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DOI: https://doi.org/10.1007/s00410-016-1251-y