The geological development of the southern part of the New England Fold Belt

Two major divisions of the New England Fold Belt, Zone A and Zone B, are separated by the Peel Fault. Deposition in these two zones was probably contemporaneous (Lower Palaeozoic ‐ Lower Permian). Terminal orogenesis in both zones was also contemporaneous (Middle Permian) but whereas in Zone A deformation was only moderate, metamorphism was of burial type, and granitic emplacement was uncommon, in Zone B many rocks were severely deformed and regionally metamorphosed, and both syn‐tectonic and post‐tectonic granites are widespread.

Pre‐orogenic palaeogeography is envisaged in terms of an evolving volcanic chain ‐ fore‐chain basin ‐ trench system, with an outer non‐volcanic arc developed in the Carboniferous. Cessation of movement on a subduction zone dipping westward beneath the volcanic chain is believed to have caused the Middle Permian deformation, but neither metamorphism nor the granitic rocks are directly related to subduction.     

Syngenetic gold in western Victoria: occurrence,age and dimensions

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.     

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites,Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ～200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.     

Detrital iron-ore deposits in the Iron Ore Group of rocks,northern Orissa,eastern India

Detrital iron deposits (DID) are located adjacent to the Precambrian bedded iron deposit (BID) of Joda near the eastern limb of the horseshoe-shaped synclinorium, in the Bonai–Keonjhar belt of Orissa. The detrital ores overlie the Dhanjori Group sandstone as two isolated orebodies (Chamakpur and Inganjharan) near the eastern and western banks of the Baitarani River, respectively. The DID occur as pebble/cobble conglomerates containing iron-rich clasts cemented by goethite. Mineralogy, chemistry and lamination of these clasts are similar to that found in the nearby BID ores. Enrichment of trace and rare-earth elements in the DID relative to the BID is attributed to their concentration during the precipitation of cementing material. The detrital iron orebodies formed when Proterozoic weathering processes eroded pre-existing BID outcrops located on the Joda Ranges, and the resulting detritus accumulated in the paleochannels. In situ dissolution in association with abundant organic material produced Fe-saturated groundwater, which re-precipitated as goethite within the aggraded channel to cement the detritals. Growth of microplaty hematite in the goethite matrix suggests some level of subsequent burial metamorphism.     

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt,southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.     

Structure and metamorphism of the Calliope Volcanic Assemblage: Implications for middle to Late Devonian orogeny in the northern New England Fold Belt

The Late Silurian to Middle Devonian Calliope Volcanic Assemblage in the Rockhampton region is deformed into a set of northwest‐trending gently plunging folds with steep axial plane cleavage. Folds become tighter and cleavage intensifies towards the bounding Yarrol Fault to the east. These folds and associated cleavage also deformed Carboniferous and Permian rocks, and the age of this deformation is Middle to Late Permian (Hunter‐Bowen Orogeny). In the Stanage Bay area, both the Calliope Volcanic Assemblage and younger strata generally have one cleavage, although here it strikes north to northeast. This cleavage is also considered to be of Hunter‐Bowen age. Metamorphic grade in the Calliope Volcanic Assemblage ranges from prehnite‐pumpellyite to greenschist facies, with higher grades in the more strongly cleaved rocks. In the Rockhampton region the Calliope Volcanic Assemblage is part of a west‐vergent fold and thrust belt, the Yarrol Fault representing a major thrust within this system.

A Late Devonian unconformity followed minor folding of the Calliope Volcanic Assemblage, but no cleavage was formed. The unconformity does not represent a collision between an exotic island arc and continental Australia as previously suggested.     

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015 ± 4.4 Ma, 966.5 ± 3.5 and 808 ± 3.1 respectively. The plagiogranite shows high SiO₂, Na₂O and extremely low K₂O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO₂ and moderate Na₂O, together with high K₂O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO₂ moderately higher Na₂O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.     

Provenance and origins of a Late Paleozoic accretionary complex within the Khangai–Khentei belt in the Central Asian Orogenic Belt,central Mongolia

We have investigated the petrography, geochemistry, and detrital zircon U–Pb LA-ICPMS dating of sandstone from the Gorkhi Formation of the Khangai–Khentei belt in the Ulaanbaatar area, central Mongolia. These data are used to constrain the provenance and source rock composition of the accretionary complex, which is linked to subduction of the Paleo-Asian Ocean within the Central Asian Orogenic Belt during the Middle Devonian to Early Carboniferous. Field and microscopic observations of the modal composition of sandstone and constituent mineral chemistry indicate that the sandstone of the Gorkhi Formation is feldspathic arenite, enriched in saussuritized plagioclase. Geochemical data show that most of the sandstone and shale were derived from a continental margin to continental island arc setting, with plutonic rocks being the source rocks. Detrital zircon ²⁰⁶Pb/²³⁸U ages of two sandstones yields age peaks of 322 ± 3 and 346 ± 3 Ma. The zircon ²⁰⁶Pb/²³⁸U age of a quartz–pumpellyite vein that cuts sandstone has a weighted mean age of 339 ± 3 Ma. Based on these zircon ages, we infer that the depositional age of sandstone within the Gorkhi Formation ranges from 320 to 340 Ma (i.e., Early Carboniferous). The provenance and depositional age of the Gorkhi Formation suggest that the evolution of the accretionary complex was influenced by the intrusion and erosion of plutonic rocks during the Early Carboniferous. We also suggest that spatial and temporal changes in the provenance of the accretionary complex in the Khangai–Khentei belt, which developed aound the southern continental margin of the Siberian Craton in relation to island arc activity, were influenced by northward subduction of the Paleo-Asian Ocean plate.     

兴蒙造山区及邻区早白垩世盆地岩石地层格架 与沉积古地理演化

在重点梳理兴蒙造山区及其相邻地区早白垩世地层分布与沉积古地理特征的基础上,从地层沉积学角度探讨了研究区早白垩世沉积盆地特征。兴蒙造山区中东部在晚侏罗世巨厚粗碎屑沉积建造之上发育了巨厚的酸性－中性火山岩－火山碎屑岩与河流－湖泊相沉积岩系,构成中国东北部巨型NE向火山岩－沉积岩带。兴蒙造山区中东部早白垩世早中期以断陷（裂谷）盆地为主,古地貌以高地、河流和湖泊共存为特征,气候温湿且炎热,热河生物群萌生;早白垩世中晚期,北东向地壳强烈伸展并进一步向外围地区扩展,沉积盆地及充填建造和热河生物群也相应地向更广泛区域辐射发展。兴安岭-燕山沉积-火山岩带与古太平洋板块俯冲无关,板内软流圈上涌导致的区域性伸展是其主要的成因动力机制,也是中国东部早白垩世中晚期更大规模区域性伸展作用的序幕。兴蒙东部早白垩世晚期沉积盆地发育和古地理格局受Izanagi板块向亚洲大陆东部俯冲弧后伸展构造机制约束。     

Variscan plate dynamics in the circum-carpathian area

Abstract

Magmatic and metamorphic events, imprinted in the crystalline rocks of the so-called core mountains inside the Alpine structure of the Inner Carpathians, allow the re-construction of the history of the Rheic Ocean opening, its development and its final closure. Intra-Carpathian core-mountains are the remnants of the continents that drifted away from Gondwana and docked, initially, with Baltica as part of Avalonia and later on as parts of the Gondwana-derived Armorica Terrane Group or as a separate micro-continent.

All magmatic suites, mafic and felsic, present in the Carpathians core mountains, show similarities to those found in the European Variscan Belt. All described- and dated metamorphic and magmatic events also have equivalents in the evolution of the Caledonian-Variscan Belts of Europe. The most pronounced feature of all Carpathian core mountains is the syn-collisional, multistage I/S granitoid magmatism (370-340 Ma) related to subduction, mafic-magma influx, extensional decompression and slab melting. That episode marked the Laurussia - Gondwana collision and closure of the Rheic Ocean, as in the whole of Central and Western Europe.

The Carpathian core-mountains, currently dispersed inside the Alpine mountain chain, can be considered the broken fragments of the eastern prolongation of the Variscan orogenic belts – possibly part of the Moldanubian Unit.