An isotopic, geochemical and structural study of the proterozoic Einasleigh Metamorphics, northern Queensland

McNaughton, Neal Jesse (1981). An isotopic, geochemical and structural study of the proterozoic Einasleigh Metamorphics, northern Queensland PhD Thesis, School of Physical Sciences, The University of Queensland. doi:10.14264/uql.2014.527

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Author McNaughton, Neal Jesse
Thesis Title An isotopic, geochemical and structural study of the proterozoic Einasleigh Metamorphics, northern Queensland
School, Centre or Institute School of Physical Sciences
Institution The University of Queensland
DOI 10.14264/uql.2014.527
Publication date 1981
Thesis type PhD Thesis
Supervisor A. F. Wilson
Total pages 614(2v.)
Language eng
Subjects 0403 Geology
Formatted abstract
The Einasleigh Metamorphics forms the basal unit of the polymetamorphic Proterozoic rocks of the Georgetown Inlier, northern Queensland. This study elucidates the metamorphic controls on the Isotopic and chemical evolution of coexisting minerals in different rock types during discrete structural-metamorphic events in the Einasleigh Metamorphics. The mineral, whole-rock and stable isotope geochemistry of mafic rocks of the Turpentine Hill area and calc-silicates of the Daintree area are examined in detail within a structural framework.

Layered mafic rocks from the Turpentine Hill area initially underwent two medium-high grade metamorphic events (M1 and M2). The oceanic tholeiite composition of the mafic rocks was not greatly affected by these events, although Fe2+/(Fe2+ + Fe3+) increased during metamorphism . M1 produced an amphibolite facies assemblage in the mafic rocks, although relict textural and compositional features of the tholeiites are preserved. Orthopyroxene formed during the M2 granulite facies event and metamorphic conditions of 814 ± 13°C (Wood-Banno) and 4-5 kb are estimated. The M2 event is dated at 1461 ±34 m.y. by the K/Ar method.

Oxygen isotope geothermometry of coexisting silicates in five mafic granulites gives inconsistent results and reflects isotopic disequilibrium. It is concluded that in these rocks (a) coexisting OPX and CPX are not in isotopic equilibrium in the presence of HBL, (b) HBL appears to be in isotopic equilibrium with OPX, (c) the 18O disequilibrium in QTZ is related to its abundance, and (d) PLAG 18O disequilibrium is variable. The oxygen isotope disequilibrium in the silicate minerals is also reflected by the cation zoning in the minerals. These features can be related to differential exchange of 18O between the silicate minerals and an evolving fluid during cooling, although other mechanisms are possible.

The stable isotope, mineral and whole-rock geochemical evidence is consistent with a fluid that is CO2-dominated at the peak of M2, but becomes H2O-rich during cooling for conditions of Pload ≥ Pfluid.

Field mapping of mafic rocks in nearby areas of the Einasleigh Metamorphics indicates granulite to amphibolite facies assemblages for M2, and yields the first delineation of the areal extent of the granulite metamorphism. The mafic rocks from each area are structurally, compositionally and petrologically similar to the mafic rocks of the Turpentine Hill area.

The calc-silicates of the Daintree area underwent at least four tectonic events. Isoclinal folding was produced by the first two events, and M2 was accompanied by amphibolite facies metamorphic conditions of 670°C and 4kb. A later event under greenschist facies conditions (M3) produced an inferred anticlinorium with a shear zone along the axial plane. The shear zone transects a pre-M2 protore body and is associated with pervasive hydration and retrogression of the M2 assemblages, epidotisation and veining. Albitisation in the calc-silicates, significant lowering of δ18O in the protores and chemical modification of the protores and the formation of late-veins are thought to be accompanied by convective circulation of meteoric groundwater through the calc-silicates and up the shear zone conduit during late-M3.
Keyword Metamorphic rocks
Geology -- Queensland, Northern
Additional Notes Other Title: Einasleigh metamorphics.

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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