Isotopic measurements in the Cape York Peninsula area, North Qeensland

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Abstract
K‐Ar and Rb‐Sr isotopic measurements have been made on the north‐south belt of igneous and metamorphic rocks from the Peninsula Ridge and Yambo Inlier of Cape York Peninsula. Four periods of Palaeozoic igneous activity appear to have been denned. These are (87’Rbλ = 1.39 X 10–11y–1) about 415 m.y., about 400 m.y., 385–390 m.y., and 255–280 m.y., with the youngest dates to the north and northeast. The largest volume of magma, the Kintore Adamellite was emplaced during the 285–390 m.y. period. Initial 87Sr/86Sr ratios range from 0.715 (a granodiorite) through 0.72–0.74 (muscovite adamellite) to 0.76 (leuco‐adamellite), which suggests a high component of old crustal material in the latter types. The host metamorphics grade from greenschist facies in the west to almandine‐amphibolite facies in the centre and south. Limited direct data suggest that the greenschists are older than 1400 m.y. This is supported by intrusive dolerite dating greater than 1800 m.y. Rocks possibly 2000 m.y. old are thus adjacent to the Australian northeast coast and place drastic limitations on the possibility of continual continental growth to the east. Rb/Sr measurements on minerals of the almandine amphibolite rocks record the major Kintore event. Total rock measurements have high uncertainties but give only slightly older figures. Initial 87Sr/86Sr ratios of these apparent total rock isochrons are high, 0.735–0.745. Gross isotopic redistribution must have occurred during the Palaeozoic metamorphism. The Rb/Sr isotopic and geochemical relationships suggest that some of the granitic rocks have been derived from the equivalent of the present greenschist facies suite, and that the almandine amphibolite facies was, in part, remetamorphosed during the Palaeozoic and is possibly partly residual after metamorphic segregation. The region has been examined from the plate tectonic point of view and shows that many of the requirements of a cordilleran‐type mountain belt of Dewey & Bird (1970) existed during the mid‐upper Palaeozoic. The Palmerville Fault and the Hodgkinson Basin are key units in this interpretation. Dolerite, possibly 2000 m.y. old, could be contemporaneous with voluminous dolerites of similar age from the Kimberley region (Australia) and of Venezuela and Guyana. They may be a useful continental breakup indicator, as are the Gondwana dolerites.

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