The spatial-Temporal distributions and migrations of Mesozoic magmatism suggest that Mesozoic igneous rocks in South China can be divided into "two regions and four belts" , which are thought to be the products of convergence of multiple blocks as well as multi-stages and multi-directional compressional-extensional orogeny. However, it is impossible that the single subduction of the paleo-Pacific plate to form multi-direction and wide( > I (KK) km) tectonic deformation and mag- matic rock belt. In this paper. We evaluated the model of the paleo-Pacific Plate subduction, which widely circulated on late Mesozoic magmatism in Cathaysian Block at present, to address the its main contribution and shortage. We put forward that approximately middle-Jurassic( 175±5 Ma) , the South China entered the paleo-Pacific tectonic system, roughly from south to north oblique subduction of the paleo-Pacific Plate. After 120 Ma, the subduction direction of paleo-Pacific Plate has changed to westward forward subduction dramatically. At the end of Mesozoic, the South China continent has become a passive margin so that there are the different tectonic setting roughly contemporary in southeastern China coast and east- em Taiwan. The former is the within-continent tectonic environment of the extensional-break up setting, where as and the latter is the arc tectonic environment with collision-compresive stress. 相似文献
Usual ductile shear zone has relatively high strain magnitude.However,for high-grade metamorphic rocks in Daqingshan area,the related strain is very low with weakly developed lineation.Significantly,the fi-nite strain type with a K-value slightly lower than 1 and is characterized by compressional strain.Because the special tectonic environment with high temperature,the percentage of melt is relatively higher,which leads to a series of microscopic phenomenon such as multi-grain quartz bands,feldspar three edges equilibrium-like tex-ture,granular flow,etc.As a consequence,the tectonites in the studied area illustrate a unique character:ob-vious rheological meso-deformation structure but with very weak intracrystalline micro-deformation. 相似文献
The Upper Permian Emmaville Volcanics at Deepwater, northeastern New South Wales, consist of a diverse succession of calc‐alkaline silicic‐intermediate ignimbrites, volcaniclastics and minor lavas. This 2.5 km‐thick sequence underlies and outcrops extensively along the northern margin of the Dundee Rhyodacite Outlier at Dundee. Detailed mapping and facies analysis have revealed eight locally mappable units namely; Magistrate Volcanic Member (rhyolitic ignimbrites), Wollundi Mudstone Member, Dellwood Ignimbrite Member, Marrawarra Rhyolite Member, Top‐Crossing Sandstone Member, Arranmor Ignimbrite Member, Yarramundi Andesite Member (lava, breccia) and Welcome Volcanic Member (rhyolitic ignimbrites). All volcanic units are contained in two fault‐bounded blocks of different lithology and structure. The volcanic succession ranges in composition from andesite to high‐silica rhyolite (58.6–78% SiO2). Chemical characteristics include enrichment in K2O (>3.5%), Al2O3 and large‐ion lithophile elements (LILE: Rb, K and light rare‐earth elements (LREE)), and depletion in high field strength elements (HFSE: Ti, Nb and Zr). These geochemical attributes reflect a continental subduction‐related signature. The facies architecture indicates that the principal volcanic features of the Late Permian palaeogeography in northeastern New South Wales was a topographically subdued depression flanked by low‐angle ignimbrite sheets with rhyolitic‐intermediate volcanic centres rising gently from the sloping terrain. The succession demonstrates that during the Late Permian andesitic volcanism was present, although localised. A modern analogue for the setting of the Emmaville Volcanics is the Quaternary Taupo Volcanic Zone (New Zealand). 相似文献
AbstractInformation, mainly from the granitic and silicic volcanic rocks in the Stawell, Bendigo and Melbourne structural zones in the state of Victoria, shows that the sources of both the S- and I-type rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with the sources of similar granitic rocks in the Melbourne Zone, consistent with the absence of the mainly Proterozoic Selwyn Block beneath most of the SBZ. Below a mid-crustal décollement in the SBZ, the crust is evidently highly variable and possibly includes thinned Proterozoic crust. There is geochronological evidence for ca 400 and ca 370?Ma granulite-grade metamorphic events here, and, after this double bout of metamorphism, and depletion in the silicic melt component, the constituents of the entire deep crust of the SBZ would have densities similar to those of overlying, much lower-grade Cambrian metabasaltic to boninitic rocks. Thus, granitic magmas may have formed here by partial melting of a variety of rock types, probably with back-arc affinities, with ages that may extend back to the Proterozoic. Therefore, the basement of the SBZ is unlikely to consist solely of thick ocean-floor rocks, as in some current interpretations.
KEY POINTS
The sources of the Devonian granitic rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with those of the Melbourne Zone granites.
Two Devonian granulite-facies events left the melt-depleted deep SBZ crust with densities similar to those of overlying Cambrian metabasaltic rocks.
The SBZ Devonian granitic magmas probably formed by partial melting of heterogeneous Proterozoic to Cambrian arc-related crust, below the mid-crustal décollement.
AbstractFour economic porphyry Cu–Au deposits and several prospects have been investigated in the Northparkes district, part of the Ordovician to early Silurian Junee–Narromine Belt of the Macquarie Arc, New whole-rock geochemical data from the Northparkes porphyry Cu–Au district, NSW, indicate that the mineralising intrusive complexes exhibit distinct arc signatures that are transitional from high-K calc-alkaline to silica-saturated alkalic. Based on ratios of Sr/Y vs Y (e.g. Sr/Y > ~20 and Y < ~17?ppm) the mineralising intrusions are interpreted to have crystallised from fractionated hydrous melts indicating the suppression of plagioclase crystallisation in favour of hydrous mineral phases. This interpretation is supported by listric-shaped rare earth element curves and the presence of primary hornblende phenocrysts indicating elevated magmatic water contents. There is an association of mineralising intrusions with a low Zr trend both in the mineralised Northparkes district intrusive rocks and in mineralised porphyry-related intrusive rocks globally. A newly developed fertility indicator ratio Zr/Y ~10% is more accurate at identifying the mineralised rocks at Northparkes than the conventional Sr/Y vs Y fertility indicator diagram, successfully identifying 92% of the mineralising intrusions, mainly owing to the fact that it is less affected by hydrothermal alteration. The insensitivity of Zr–Y to alteration makes this indicator a useful new tool that may lead to enhanced probabilities for future discoveries in the Northparkes district, broader Macquarie Arc and altered rocks globally.
KEY POINTS
Mineralising intrusions in the Northparkes district have distinct Zr vs Y concentrations.
The Zr vs Y indicator of magmatic fertility is less sensitive to alteration than Sr-based indicators.
The Zr vs Y magmatic fertility indicator identified at Northparkes is not unique and identifies mineralising intrusions in other porphyry fields.