University of Tasmania
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Submarine volcanism and alteration in the Cambrian, northern Central Volcanic Complex, western Tasmania

posted on 2023-05-26, 05:20 authored by Gifkins, CC
The Cambrian northern Central Volcanic Complex is a submarine volcanic succession within the Mount Read Volcanics in western Tasmania, Australia. It comprises a relatively complex assemblage of originally glassy and crystalline, rhyolitic to basaltic, lavas, syn-volcanic intrusions and syn-eruptive volcaniclastic facies which host massive sulfide mineralisation. Diagenesis, hydrothermal alteration, regional greenschist facies metamorphism and deformation have all modified the volcanic succession. In this study, the integration of geological mapping, volcanic facies and structural analyses, and lithogeochemistry is used to divide the northern Central Volcanic Complex jnto four distinctive formations: the Sterling Valley Volcanics, Mount Black Formation, Kershaw Pumice Formation and the Hercules Pumice Formation. These formations replace various informal stratigraphic units in the northern Central Volcanic Complex: the Sterling Valley Volcanics, Mount Black Volcanics, and the Footwall Pyroclastics and Host Rocks of the Rosebery-Hercules host sequence. The Sterling Valley Volcanics are the oldest exposed part of the northern Central Volcanic Complex. They are a thick succession (> 1.5 km thick) comprising dacitic to basaltic lavas and sills, and polymictic mafic volcaniclastic facies. The polymictic mafic volcaniclastic facies were derived from the resedimentation of locally available clasts of quench brecciated basalt, dacite, pillow fragments and scoria. The Sterling Valley Volcanics represent the proximal to medial facies of a submarine tholeiitic basaltic volcanic centre that formed during a period of extension and back-arc basin development. Intercalated calc-alkaline dacites attest to coeval tholeiitic and calc-alkaline volcanism during formation of part of the Sterling Valley Volcanics. The Mount Black Formation (previously part of the Mount Black Volcanics) is a thick succession (>1.6 Inn) of mainly massive, flow-banded and autobrecciated felsic lavas, domes, cryptodomes and syn-volcanic sills. It represents the proximal facies of a calc-alkaline dacitic to rhyolitic, mainly effusive and intrusive volcanic complex. The Kershaw Pumice Formation consists of pumice-rich facies and associated rhyolitic and dacitic lavas and sills that were previously assigned to the Mount Black Volcanics. The Hercules Pumice Formation refers to the Footwall Pyroclastics and Host Rocks in the Rosebery-Hercules host sequence. The Kershaw and Hercules Pumice Formations have been correlated based on lithofacies and compositional similarities and structural interpretation. These formations (>800 m thick) comprise successions of pumice breccia, pumice-rich sandstone and shard-rich siltstone, intercalated with pumicelithic clast-rich breccia and sandstone, and rhyolitic and dacitic lavas and syn-volcanic intrusions. The laterally extensive pumice-rich facies are the products of a large volume (>24 km3), rhyolitic explosive eruption and were deposited proximal to the vent from water-supported density current and suspension. Pumiceous hyaloclastite and peperite are associated with the margins of rhyolitic lavas and sills that flowed over or intruded wet, unconsolidated pumice breccia, pumice-rich sandstone, shard-rich siltstone and black mudstone. Pumice-lithic clast-rich breccia and sandstone were generated by resedimentation of unconsolidated felsic hyaloclastite, autobreccia and pumice-rich deposits. The Kershaw and Hercules Pumice Formations and the Mount Black Formation are interpreted to be the explosive and effusive products of a single calc-alkaline felsic volcanic centre. Tholeiitic basalts of the Henty Dyke Swarm subsequently intruded the northern Central Volcanic Complex. These dykes are compositionally similar to tholeiitic basalts of the Sterling Valley Volcanics and suggest that extension-related, mantle-derived magmatism was reactivated after formation of the Central Volcanic Complex. The Hangingwall Volcaniclastics, previously considered part of the northern Central Volcanic Complex, are now correlated with the White Spur Formation and thus this unit is included in the Dundas Group. This new stratigraphy for the northern Central Volcanic Complex has significant implications for VHMS exploration in the Mount Read Volcanics. The correlation of the Kershaw and Hercules Pumice Formations suggests that there is potential for the Host Rocks to the Rosebery and Hercules ore deposits (Host-rock member) to be repeated east of the Mount Black Fault. The stratigraphy of the northern Central Volcanic Complex is disrupted and repeated by Middle to Late Cambrian faults and Devonian faults and folds, resulting in substantial thickening of the Central Volcanic Complex between Rosebery and Tullah. Metamorphosed diagenetic and hydrothermal alteration facies in the northern Central Volcanic Complex have been discriminated using a combination of alteration mineralogy, overprinting relationships, texture, distribution, intensity and whole-rock geochemistry. Alteration facies that are interpreted to be the metamorphosed equivalents of diagenetic clays and zeolites are regionally extensive and overprinted by the regional Devonian cleavage. Although widespread, locally their distributions are patchy and their intensities are variable, reflecting the complexity of the original volcanic textures. Thin films of sericite, carbonate and chlorite-sericitehematite replaced clays that coated original glassy surfaces at the onset of diagenesis. Feldspar-quartzsericite, chlorhe-sericite and chlorite-sericire-hematite alteration facies replaced zeolites and clays that filled pore space and altered glass, prior to and synchronous with diagenetic compaction. In pumice rich facies, a bedding-parallel stylolitic foliation reflects the dissolution of glass during compaction. Fiamme are interpreted to be the product of diagenetic alteration and compaction of pumice clasts. Development of the final diagenetic alteration facies, chlorite-epidote, commenced after compaction and continued during Devonian regional metamorphism. Diagenetic alteration involved significant mineralogical and textural changes but only minor changes in composition. These changes are consistent with the interaction of rhyolitic and basaltic glass with seawater during burial. In contrast, alteration facies interpreted to be hydrothermal are local in distribution, cross-cut stratigraphic boundaries and involved significant changes in texture, mineralogy and composition. Diagenetic alteration in the northern Central Volcanic Complex was widespread, reached high temperature diagenetic mineral assemblages (albite-quartz-sericite and chlorite-epidote) and formed thick (>2 km) alteration zones. These alteration zones reflect a high-grade diagenetic alteration system and elevated geothermal gradient within the very thick proximal, submarine succession dominated by glassy volcanic facies.


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