P-T-t- evolution across a Paleozoic greenschist to amphibolite facies transition in Greenland Group rocks in the Collie Range, south Westland, New Zealand.
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Abstract:
A superb prograde metamorphic sequence in the Greenland Group is exposed in the Collie Range in south Westland. Metamorphic grade progressively increases over a 6 km transect, from lower greenschist facies slates and greywackes near the coast to amphibolite facies gneisses immediately northwest of the Alpine Fault. Four textural and mineralogical zones are defined. These span chlorite-sericite-albite to microclineandesine/ labradorite assemblages and record progressive recrystallization throughout. Whole rock analyses indicate mobilization of CaO, K2O, Sr, Rb Ba accompanies progressive dewatering with increasing grade. Textural zone I rocks are greenschist facies argillites and greywackes that were folded into macroscopic folds and contain an S1 schistosity. In Textural Zone II rocks, which span upper greenschist to lower amphibolite facies but contain abundant detrital minerals, porphyroblastic biotites overprint S1. Textural zone III records near complete recrystallization and weak crenulation cleavage. Textural zone IV rocks are completely recrystallized and commonly contain metamorphic segregations. In TZ III and IV amphibolites, two generations of muscovite occur. Synkinematic muscovites define a penetrative foliation that is overprinted by coarse poikiloblastic muscovites that record rehydration-mediated retrogression of microcline. Small granitoid stocks and dikes intrude TZIV rocks in the southeast of the range. The metamorphic textures observed are interpreted to show that an early regional axial-planar S1 cleavage observed in TZ I and II was transposed by S2 fabrics during peak metamorphism, causing formation of TZ III and IV. P-T conditions for the peak metamorphic assemblages recorded in TZ III and IV were obtained by a combination of Ti in biotite geothermometry and phengite geobarometry and indicate attainment of c. 620 ?C and c. 4 kbar in TZ IV and c. 540 ?C and c. 3.5 kbar in TZ III. The timing of peak metamorphism is constrained by monazite U-Pb dating to be 350-360 Ma. The change in the metamorphic fabrics and assemblages is interpreted to record initial shortening and macroscopic folding (TZ I) at >371 Ma, followed by an elevated heat-flow at c. 350-360 Ma, perhaps due to lithospheric thinning. The low P/T conditions and association with peraluminous granitoids suggest an extensional tectonic setting for peak metamorphism. Porphyroblasts athwart the fabric in TZII, III and IV are interpreted to record a later thermal perturbation associated with intrusion emplacement. The metamorphic sequence in the Collie Range represents a tilted section that was exhumed in the Cretaceous as well as during the Neogene, with the highest grades exposed closest to the Alpine Fault.
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141 pages A4, A3 map in back pocket
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2014Belton
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POLYGON ((169.419743820165621 -43.74381857453087,169.341963275031674 -43.785420738946378,169.298122091886142 -43.733983844075752,169.361254491362672 -43.700520846565034,169.419743820165621 -43.74381857453087))
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Citation
Belton, Alexis, “P-T-t- evolution across a Paleozoic greenschist to amphibolite facies transition in Greenland Group rocks in the Collie Range, south Westland, New Zealand.,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/568.