Stratigraphy and correlation of Carboniferous ignimbrites,Rocky Creek region,Tamworth Belt,Southern New England Orogen,New South Wales

Carboniferous (Visean to Westphalian) pyroclastics and lava flows in the Rocky Creek region, used to redefine the base of the Kiaman reversal, are formally defined or redefined and the status of the main formations clarified. These units include the Caroda Formation, containing the Kooringal Dacite, Boomi Rhyolite and Barney Springs Andesite Members; the Clifden Formation with the Wanganui Andesite, Glen Idle Rhyolite, Appleogue Dacite, Bexley Rhyolite, Pine Cliffs Rhyolite and Downs Rhyodacite Members; Rocky Creek Conglomerate with the Hazelvale Rhyodacite, Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members; and Lark Hill Formation with the Eulowrie Pyroclastic, Tycannah Rhyodacite and The Tops Rhyolite Members; a number of informal units are also described. The restriction of most volcanic units to one of the three thrust blocks (Boomi, Kathrose and Darthula blocks) of the Rocky Creek region, suggests their current relationships reflect either shortening due to overthrusting or an original distribution affected by depositional or erosional processes. A westerly increase in the proportion of ignimbrites indicates nearness to sources in that direction. Intermediate volcanism, largely confined to southern and central parts of the Boomi block in the east, began in the Visean and ended in the early Namurian. Acid volcanism also began in the Visean in the northern Boomi block but, with the exception of the Peri Rhyolite Member of the Clifden Formation, did not become widespread until later in the Namurian and Westphalian. In contrast, only acid volcanism took place during the early Namurian to Westphalian in the Kathrose and Darthula blocks. Correlations based on AS3 and SL13 SHRIMP dates illustrate a discordance of about 3% when compared with the most likely location for the base of the Kiaman reversal. The bases of both the Rocky Creek Conglomerate and Lark Hill Formation appear to be slightly diachronous.     

Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen,Australia

The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east–west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east–west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation during the Early Permian. This model differs to previously proposed reconstructions where curvature largely formed by orogen parallel strike-slip transportation during the Hunter Bowen Event.     

Early Permian strike-slip basin formation and felsic volcanism in the Manning Group,southern New England Orogen,eastern Australia

The Manning Group is characterised by rapidly filled strike-slip basins that developed during the early Permian along the Peel--Manning Fault System in the southern New England Orogen. Typically, the Manning Group has been difficult to date owing to the lack of fossiliferous units or igneous rocks. Thus, the timing of transition from an accretionary convergent margin in the late Carboniferous to dominantly strike-slip tectonic regimes that involved development and emplacement of the Great Serpentinite Belt (Weraerai terrane) is not well constrained. One exception are rhyolites of the Ramleh Volcanics that were erupted into the Echo Hills Formation. These developed along the dextral Monkey Creek Fault splay east of the Peel--Manning Fault System. Zircons extracted from the Ramleh Volcanics yield a U–Pb (SHRIMP) age of 295.6?±?4.6?Ma that constrains the minimum age of deposition in this basin to earliest Permian. Whole-rock geochemistry indicates these are peraluminous felsic melts enriched in LREE and incompatible elements with strong depletions in U, Nb, Sr and Ti. These are similar in age and composition to the nearby S-type Bundarra and Hillgrove plutonic supersuites. We suggest that extensive movement along the east-dipping Peel--Manning Fault System was responsible, not only for strike-slip basin development at the surface (Manning Group), but was also the locus for crustal melting that was responsible for generating S-type felsic melts that utilised hanging-wall fault splays as conduits to the surface or to coalesce in the crust as batholiths exclusively to the east of the Peel--Manning Fault System.     

东昆仑五龙沟地区早泥盆世双峰式侵入岩年代学、地球化学及其地质意义

东昆仑造山带东段发现一套早泥盆世双峰式侵入岩岩体组合.本文对该套双峰式侵入岩开展了岩石学、锆石U-Pb年代学、岩石地球化学、Sr-Nd及Hf同位素研究,结果显示:(1)该套双峰式侵入岩主要由橄榄辉长岩、辉长岩和碱长花岗岩组成,其定年结果分别为410.3±2.7Ma、409.3±2.7Ma和410.5±1.8Ma,表明其...     

Geology and geochronology of the Emu Creek Block (northern New South Wales,Australia) and implications for oroclinal bending in the New England Orogen

The southern part of the New England Orogen exhibits a series of remarkable orogenic bends (oroclines), which include the prominent Z-shaped Texas and Coffs Harbour oroclines. The oroclines are defined by the curvature of Devonian–Carboniferous forearc basin and accretionary complex rock units. However, for much of the interpreted length of the Texas Orocline, the forearc basin is mostly concealed by younger strata, and crops out only in the Emu Creek Block in the eastern limb of the orocline. The geology of the Emu Creek Block has hitherto been relatively poorly constrained and is addressed here by presenting new data, including a revised geological map, stratigraphic sections and new detrital zircon U–Pb ages. Rocks of the Emu Creek Block include shallow-marine and deltaic sedimentary successions, corresponding to the Emu Creek and Paddys Flat formations, respectively. New detrital zircon U–Pb data indicate that these formations were deposited during the late Carboniferous and that strata were derived from a magmatic source of Devonian to Carboniferous age. The sedimentary provenance and detrital zircon age distribution suggest that the sequence was deposited in a forearc basin setting. We propose that the Emu Creek and Paddys Flat formations are arc-distal, along-strike correlatives of the northern Tamworth Belt, which is part of the forearc basin in the western limb of the Texas Orocline. These results confirm the suggestion that Devonian–Carboniferous forearc basin rocks surround the Texas Orocline and have been subjected to oroclinal bending.     

MORB‐like rocks in a Palaeozoic convergent margin setting,northeast New South Wales

The Devonian mafic rocks from the Folly Basalt, northeast New South Wales, were emplaced in the forearc section of the Devonian‐Carboniferous magmatic arc preserved in the western part of the New England Fold Belt. Trace‐element abundances in fractionated metadolerites (maximum concentration of Ni = 85 ppm) from the Folly Basalt outcropping near Nundle demonstrate that these rocks have MORB affinity. Chondrite‐normalised rare‐earth element patterns are smooth and quasi‐horizontal; Ce/Yb ratios are 3.34–7.98; (La/Yb)_N ratios range from 0.69 to 2.23; (La/Sm)_N ratios of the rocks range from 0.63 to 1.55. The data are compatible with an origin of the melts from large degrees (>15%) of partial melting of mantle peridotite. A plausible mechanism for the production and emplacement of depleted magmas in the forearc zone of the Middle Palaeozoic eastern Australian magmatic arc involves the subduction of a hot oceanic spreading centre, which could cause the presence of a region of asthenospheric temperatures below the upper plate. It is also suggested that sustained high‐temperature conditions may have prevailed in the eastern Australian mantle for at least the last 400 million years.     

Structural fabric evidence for indentation tectonics in the Nambucca Block,southern New England Fold Belt,New South Wales

Structural studies of Lower Permian sequences exposed on wave‐cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle — ductile/brittle events are recorded in the lower grade (sub‐greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle — ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west‐southwest‐trending, open, south‐southeast‐verging, inclined folds (F₁ ⁰) at Grassy Head in the south, to east‐northeast‐trending, recumbent, isoclinal folds (F₁ ⁰; F₂ ⁰) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D₁ in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D₁ and D₂ in the higher grade rocks. South‐ to south‐southwest‐directed tectonic transport and north‐south shortening operated during these earlier events. Subsequently, north‐northeast‐trending, open, upright F₃ ² folds and inclined, northwest‐verging, northeast‐trending F₄ ² folds developed with poorly to moderately developed axial planar, crenulation cleavage (S₃ and S₄) formed by solution transfer processes. These folds formed heterogeneously in S₂ throughout the higher grade areas. Later northeast‐southwest shortening resulted in the formation of en échelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east‐northeast‐west‐southwest during later north‐northeast to northeast, dextral, strike‐slip faulting and then to approximately northwest‐southeast during the formation of east‐southeast to southeast‐trending, strike‐slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S₁ trends east‐west and dips moderately to the north in areas unaffected by later events. S₂ has a similar trend to S₁ in less‐deformed areas, but is refolded about east‐west axes during D₃. S₃ is folded about east‐west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south‐directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east‐west‐trending structures. The effects of the Coffs Harbour Block were greatest during D₁ and D₂.     

The Keepit arc: provenance of sedimentary rocks in the central Tablelands Complex,southern New England Orogen,Australia, as recorded by detrital zircon

Detrital zircon from the Carboniferous Girrakool Beds in the central Tablelands Complex of the southern New England Orogen, Australia, is dominated by ca 350–320 Ma grains with a peak at ca 330 Ma; there are very few Proterozoic or Archean grains. A maximum deposition age for the Girrakool Beds of ca 309 Ma is identified. These data overlap the age of the Carboniferous Keepit arc, a continental volcanic arc along the western margin of the Tamworth Belt. Zircon trace-element and isotopic compositions support petrographic evidence of a volcanic arc provenance for sedimentary and metasedimentary rocks of the central Tablelands Complex. Zircon Hf isotope data for ca 350–320 Ma detrital grains become less radiogenic over the 30 million-year record. This pattern is observed with maturation of continental volcanic arcs but is opposite to the longer-term pattern documented in extensional accretionary orogens, such as the New England Orogen. Volcanic activity in the Keepit arc is inferred to decrease rapidly at ca 320 Ma, based on a major change in the detrital zircon age distribution. Although subduction continues, this decrease is inferred to coincide with the onset of trench retreat, slab rollback and the eastward migration of the magmatic arc that led to the Late Carboniferous to early Permian period of extension, S-type granite production and intrusion into the forearc basin, high-temperature–low-pressure metamorphism, and development of rift basins such as the Sydney–Gunnedah–Bowen system.     

Tectonic cycles of the New England Orogen,eastern Australia: A Review

The New England Orogen (NEO), the youngest of the orogens of the Tasmanides of eastern Australia, is defined by two main cycles of compression–extension. The compression component involves thrust tectonics and advance of the arc towards the continental plate, while extension is characterised by rifting, basin formation, thermal relaxation and retreat of the arc towards the oceanic plate. A compilation of 623 records of U–Pb zircon geochronology rock ages from Geoscience Australia, the geological surveys of Queensland and New South Wales and other published research throughout the orogen, has helped to clarify its complex tectonic history. This contribution focuses on the entire NEO and is aimed at those who are unfamiliar with the details of the orogen and who could benefit from a summary of current knowledge. It aims to fill a gap in recent literature between broad-scale overviews of the orogen incorporated as part of wider research on the Tasmanides and detailed studies usually specific to either the northern or southern parts of the orogen. Within the two main cycles of compression–extension, six accepted and distinct tectonic phases are defined and reviewed. Maps of geological processes active during each phase reveal the centres of activity during each tectonic phase, and the range in U–Pb zircon ages highlights the degree of diachronicity along the length of the NEO. In addition, remnants of the early Permian offshore arc formed during extensive slab rollback, are identified by the available geochronology. Estimates of the beginning of the Hunter-Bowen phase of compression, generally thought to commence around 265?Ma are complicated by the presence of extensional-type magmatism in eastern Queensland that occurred between 270 and 260?Ma.     

Metamorphism in the New England Orogen,eastern Australia: a review

Abstract

This paper summarises current knowledge on metamorphism within the entire New England Orogen (NEO) of eastern Australia. Rocks recording metamorphic assemblages characteristic of each of the three metamorphic facies series (high, medium and low P/T) have been identified within the orogen. These include high P/T blueschists and eclogites, mid P/T orogenic metamorphism and low P/T contact aureoles and sub-regional high-temperature–low-pressure (HTLP) metamorphism (regional aureoles). Metamorphism is described as it relates to six tectonic phases of development of the NEO that together comprise two major cycles of compression–extension. Medium–high-grade contact metamorphism spans all six tectonic phases while low-grade burial and/or orogenic metamorphism has been identified for four of the six phases. In contrast, exposure of high P/T eclogites and blueschists, and generation of sub-regional low P/T metamorphism is restricted to extensional phases of the orogen. Hallmarks of the orogen are two newly identified zones of HTLP metamorphism, the older of which extends for almost the entire length of the orogen.

1. KEY POINTS

2. The orogen is dominated by low-temperature rocks while high-temperature amphibolite to granulite facies rocks are restricted to small exposures in HTLP complexes and contact aureoles.

3. Blueschist metamorphism falls into two categories; that associated with subduction during the Currabubula-Connors continental arc phase occurring at depths of ～13–30?km; and the other of Cambrian–Ordovician age, exposed within a serpentinite melange and associated with blocks of eclogite. The eclogite, initially from depths of ～75–90?km, appears to have been entrained in the deep crust for an extended period of geological time.

4. A comprehensive review of contact metamorphism in the orogen is lacking and as studies on low-grade metamorphism are more extensive in the southern part of the orogen than the north, this highlights a second research gap.

     

Hydrothermal alteration and mineralisation of the Glen Eden Mo-W-Sn deposit: a leucogranite-related hydrothermal system,Southern New England Orogen,NSW, Australia

The Glen Eden Mo-Sn-W deposit in north-eastern New South Wales, Australia, is an example of a leucogranite-related, low-grade, large-tonnage hydrothermal system. It occurs in the southern part of the New England Orogen and is hosted within Permian felsic volcanic rocks, intruded at depth by dykes of porphyritic microleucogranite (Glen Eden Granite). The deposit is hosted within a pipe-like quartz-rich greisen breccia body about 500 m in diameter, surrounded by a greisen zone several hundred metres across, zoning out into altered volcanic rocks. The dominant ore minerals, largely hosted as open space fillings and disseminations in quartz and quartz-rich greisen, are molybdenite, wolframite and cassiterite; they are accompanied by minor to trace amounts of muscovite, fluorite, topaz, siderite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, bismuth, bismuthinite, joseite A, cosalite, galenobismutite, beryl, anatase and late-stage dickite and kaolinite. Two types of breccia are recognised: (1) greisenised volcanic rock fragments (quartz + muscovite), cemented by hydrothermal quartz ± K-feldspar ± ore minerals, and (2) fragments of hydrothermal quartz ± cassiterite ± wolframite enclosed in quartz ± clay. In both types of breccia and in stockwork veins, there is evidence of early precipitation of Mo-Sn-W phases, followed by Bi minerals and base metal sulfides (± fluorite, siderite).Breccia formation and associated hydrothermal alteration (greisen, potassic, argillic, propylitic) are interpreted to be related to devolatilisation of the highly fractionated Glen Eden Granite of early Triassic age (240±1 Ma based on ⁴⁰Ar/³⁹Ar geochronology of greisen muscovite) as well as to fluid mixing with meteoric waters. The breccia pipe could have formed in part by rock dissolution and collapse, as well as by explosive degassing of boiling fluids. Fluid inclusion evidence is consistent with boiling, with breccia pipe formation and mineralisation having mainly occurred at 250–350 °C from fluids with salinity of 0.4–9 wt% NaCl equivalent in the dilute types and 30–47 wt% NaCl equivalent in the hypersaline types. Stable isotopic evidence (O, D, C, S) indicates a strong magmatic contribution to the hydrothermal fluids and metals in the breccia. The ¹⁸O values of quartz decrease outward from the breccia pipe (10.6–12.3 in the pipe to 3.4–8.7 in the peripheral quartz) indicating that there has been mixing with isotopically light (high latitude) meteoric fluids, mainly after formation of the breccia pipe.     

Paleomagnetism of the Carboniferous Gresford Block,Tamworth Belt,southern New England Orogen: minor counter-clockwise rotation of a primary arc segment

Abstract

Four oroclinal structures have been identified from structural, magnetic and gravity trends across a Carboniferous continental arc, forearc basin [Tamworth Belt (TB)] and conjugate accretionary complex in the southern New England Orogen (SNEO) of eastern Australia. None of the structures has yet been confirmed conclusively by paleomagnetism as oroclinal. Ignimbrites are common within the forearc basin and have been demonstrated to retain primary magnetisations despite prevalent overprinting. They are well exposed across six major tectono-stratigraphic blocks with partly interlinked stratigraphies, making the forearc basin highly prospective to oroclinal testing by comparing pole path segments for individual blocks across curved structures. Paleomagnetic studies have shown no noticeable rotation across the western/southwestern TB (Rocky Creek, Werrie and Rouchel blocks), but documented herein is a minor counter-clockwise rotation of the Gresford Block of the southern TB. This study details paleomagnetic, rock magnetic and magnetic fabric results for 87 sites (969 samples) across the southern Gresford Block. Predominantly thermal, also alternating field and liquid nitrogen, demagnetisations show a widely present low-temperature overprint, attributed to regional late Oligocene weathering, and high-temperature primary and overprint components residing in both mainly magnetite and mainly hematite carriers. Subtle, but systematic, directional differences between magnetite and hematite subcomponents show the latter as the better cleaned, better-defined, preferred results, detailing nine primary poles of middle and late Carboniferous ages and Permian and Permo-Triassic overprints as observed elsewhere in the western/southwestern TB. The primary poles update a poorly defined mid-Carboniferous section of the SNEO pole path and demonstrate counter-clockwise rotation, quantified at about 15° ± 13° from comparison of mid-Carboniferous Martins Creek Ignimbrite Member poles for the Rouchel and Gresford blocks, that may not necessarily have been completed prior to the Hunter–Bowen phase of the Gondwanide Orogeny. This minor counter-clockwise rotation of the Gresford Block accentuates a primary curvature of the southwestern/southern TB and heralds further, more complex, rotations of the Myall Block of the southeastern TB.     

东昆仑造山带东段早泥盆世侵入岩的成因及其对早古生代造山作用的指示

研究发现,早泥盆世东昆仑造山带东段和西段均发育较大规模的岩浆活动。然而对于其成因、深部动力学机制以及与始特提斯构造演化的关联等问题一直缺乏系统的研究。本文以东昆仑东段地区跃进山岩体为例,探讨早泥盆世岩浆活动机理及其构造意义。该岩体主体由二长花岗岩和花岗闪长岩组成,含少量的辉长岩和含堇青石花岗岩,前三种岩性是本文的重点。其中,二长花岗岩和花岗闪长岩无角闪石和堇青石,具有高硅和钾,低铁、镁和钙,铝饱和指数(A/CNK)多数在1.0～1.1之间,富集大离子亲石元素(LILE:Rb、Th和K)和轻稀土(LREE),明显亏损Ba、Sr、Nb、Ta、Ti、P和Eu等元素,属过铝质I-S过渡型花岗岩;辉长岩具有显著高的Fe和Ti(FeOT为8.17%～12.70%,TiO2为4.50%～6.54%)含量,高Cu(11.5×10-6～30.6×10-6)和Cu/Ni值(1.41～6.41),相对富集的LREE((La/Yb)N为1.94～3.15,LREE/HREE为2.65～3.48),相对低的Mg#(48～50)、Cr(3.8×10-6～60.4×10-6)、Ni(1.8×10-6～12.5×10-6)值,相对于原始地幔具有明显的Nb-Ta-Ti正异常。二长花岗岩和花岗闪长岩均具有相对高的ISr值(分别为0.710～0.740和0.710)、相对低的εNd(t)值(分别为-4.05～-5.80和-3.54～-3.71)和偏古老的t2DM(分别为1.47～1.62Ga和1.43～1.45Ga),但花岗闪长岩具有相对高的εHf(t),其变化在-3.00～0.86。跃进山辉长岩具有相对高的ISr(0.711～0.714)、相对低的εNd(t)值(-3.44～-6.82)和较为集中的εHf(t)值(-2.19～1.05),显示富集地幔的特征。本次利用LA-ICP-MS锆石U-Pb定年方法获得花岗闪长岩的形成年龄为407±3Ma,辉长岩的形成年龄为406±3Ma。综合岩石学、地球化学和Sr-Nd-Hf同位素等多方面的指标,可以判断该岩体的形成为:幔源岩浆上侵至地壳,供热诱发古老的地壳物质部分熔融产生S型岩浆最终形成含堇青石花岗岩,同时与壳源熔体发生混合产生I-S过渡型岩浆并经历较高程度的分异最终形成二长花岗岩和花岗闪长岩。跃进山与东昆仑造山带其他地区早泥盆世岩浆活动均具有与典型后碰撞岩浆作用类似的岩石组合,并且显示很强的幔源岩浆作用的印记。这表明,至少从早泥盆世开始,东昆仑地区已经进入后碰撞的伸展阶段。综合区域上的研究成果,本文认为早泥盆世应为中央造山带(特别是东昆仑、北秦岭和柴北缘)始特斯构造体制转换的关键时期,这一时期相关地体的碰撞拼合已基本完成,区域构造体制由挤压开始转向伸展。     

新疆南天山泥盆纪钙碱性侵入岩的形成时代、地球化学特征及构造意义

南天山位于中亚造山带南部,塔里木盆地北缘。长期以来,有关其在古生代时期的演化规律、南天山古洋盆的俯冲方式、俯冲作用规模、持续时间及闭合时限等科学问题研究都是地学界的热点。本研究报道了出露于南天山造山带南缘(塔里木克拉通北缘)的咔拉吾勒及欧西达坂古生代钙碱性侵入岩的锆石U-Pb-Hf同位素及地球化学数据,其中咔拉吾勒细粒闪长岩、欧西达坂花岗闪长岩及钾长花岗岩的年龄分别为413.2±5.5Ma、407.3±5.1Ma及409.3±5.2Ma,都形成于早泥盆世。结合前人对研究区欧西达坂石英闪长岩的年代学(418.4±2.2Ma)及地球化学数据,确定了泥盆纪时期南天山南缘发育一套钙碱性的闪长岩-石英闪长岩-花岗闪长岩-花岗岩的岩石组合,各岩体表现出相近的时空分布及地球化学特征,判断各岩石应为同源岩浆结晶分异的产物。欧西达坂花岗闪长岩的锆石Lu-Hf同位素数据显示出较大范围的176Hf/177Hf比值(0.282185~0.282901)、εHf(t)值(-12.0~12.9)及t DM2模式年龄(573~2149Ma),体现出新老地壳物质混染的特征,指示其母岩浆为混合岩浆。结合各岩石富集大离子亲石元素(LILE)(如:Rb、Ba、Th、Sr),亏损高场强元素(HFSE)(如:Nb、Ta、Zr、Hf)及低Nb/Ta比值等特征,本研究推断该岩石组合为弧岩浆岩系列,其母岩浆可能产生于俯冲消减构造环境下新生玄武质岩浆与老陆壳重熔酸性岩浆的混合,并经历了一定的结晶分异而成岩。进一步的综合分析表明,南天山南缘古生代中期一系列岩浆活动的时代至少从晚奥陶世一直持续到晚石炭世-早二叠世,而南天山古洋盆至少于奥陶纪期间向北(伊犁-中天山地块)发生单向俯冲。晚奥陶世-泥盆纪期间,其东、西两段的俯冲极性或有所不同,西段或为持续的北向俯冲,东部地区为向南北双向俯冲,塔里木北缘在该侧转变为活动大陆边缘。本研究旨在为南天山造山带古生代期间的构造演化提供一定的约束条件。     

Devonian supra-subduction zone setting for the Princhester and Northumberland Serpentinites: implications for the tectonic evolution of the northern New England Orogen

The Princhester Serpentinite of the Marlborough terrane of the northern New England Orogen is a remnant of upper mantle peridotite that was partially melted at an oceanic spreading centre at 562 Ma, and subsequently interacted with Late Devonian island arc basalts in an intra-oceanic supra-subduction zone (SSZ) setting. The full range of rare-earth element (REE) contents, including U-shaped patterns, can be explained by a single process of reaction of partially melted, depleted peridotite with Late Devonian calc-alkaline and island arc tholeiite magmas by equilibrium porous flow, fractionating the REE by a chromatographic column effect. The Northumberland Serpentinite on South Island of the Percy Group has similar REE and high field strength element (HFSE) contents to the most depleted samples of the Princhester Serpentinite, supporting a common origin. However, spinel compositions suggest that the Northumberland Serpentinite interacted with boninitic magmas. The REE and mineral geochemistry indicates that the Princhester and Northumberland Serpentinites both represent part of the mantle component of a disrupted SSZ ophiolite. The ophiolite is considered to have formed above an east-dipping subduction zone, based on the geochemistry of Devonian island arc basalts between Mt Morgan and Monto, which include compositions identical to dykes and gabbroic blocks within the Princhester Serpentinite. Blockage of the subduction zone by collision with the Australian continent during the Late Devonian led to slab breakoff and the reversal of subduction direction, trapping the Late Devonian ophiolite in a forearc position. Its location, in a forearc setting above a growing accretionary wedge, conforms to the definition of a Cordilleran-type ophiolite. This interpretation is consistent with current views that most ophiolites are formed from young, hot and thin oceanic lithosphere at forearc, intra-arc and backarc spreading centres in a SSZ setting, and that emplacement follows genesis by 10 million years or less. Late Devonian crustal growth may have been widespread in the New England Orogen, because the disrupted ophiolite assemblage of the Yarras complex in the southern New England Orogen is probably of this age. Extensional tectonism at the end of the Carboniferous dismembered the Princhester – Northumberland ophiolite, removed the crustal section, and produced windows of accretionary wedge rocks within the fragmented ophiolite. The Princhester Serpentinite, together with fault slices of metasedimentary rocks, was thrust westward as a flat sheet over folded strata of the Yarrol Forearc Basin by a Late Permian out-of-sequence thrust during the Hunter – Bowen Orogeny, completing the emplacement of the Marlborough terrane. The Princhester and Northumberland Serpentinites could have been displaced by strike-slip movement along the Stanage Fault Zone or an equivalent structure. There is no record in the northern New England Orogen of SSZ ophiolites and volcanic arc deposits of Cambrian age, as exposed along the Peel Fault. Partial melting of the Princhester Serpentinite at an oceanic spreading centre at 562 Ma, recorded by mafic intrusives displaying N-MORB chemistry, was an earlier event that was outboard of any Early Paleozoic subduction zone along the margin of the Australian continent, and cannot be regarded as representing the early history of the New England Orogen. It is possible that the formation of intra-oceanic arcs in latest Silurian and Devonian time was the first tectonic event common to both the southern and northern New England Orogen.     

秦岭造山带东段秦岭岩群的年代学和地球化学研究

对东秦岭地区的陕西省洛南县、宁陕县、长安县和河南省淅川县出露的四个秦岭岩群变质岩进行的岩石学和地球化学研究表明,样品主要由变质火山岩和变质沉积岩组成.详细的锆石U-Pb定年结果显示三个正变质岩均形成于新元古代早期(971～843Ma),而副变质岩中富集大量新元古代碎屑锆石,根据最年轻的谐和年龄(859Ma)和早古生代的变质年龄,推测其沉积时代为新元古代中晚期.因此,北秦岭南部的秦岭岩群的变质岩主要由新元古代早期的火成岩和新元古代中晚期的沉积岩组成.变质作用主要发生在加里东期,局部有燕山期的变质作用叠加.指示北秦岭的造山作用主要发生在早古生代.岩石地球化学研究还显示秦岭岩群的新元古代火山岩均形成于火山弧构造环境,沉积岩沉积于大陆弧-活动大陆边缘环境,指示秦岭造山带在新元古代早期是一个火山弧.秦岭岩群的火山岩和沉积岩在形成时代和构造环境方面与扬子克拉通西缘的特征非常相似,表明位于北秦岭造山带的秦岭岩群应归属于扬子克拉通陆块,是扬子北缘的一个大陆边缘弧.     

Paleomagnetism of the Carboniferous Rouchel Block,Tamworth Belt,and pole path revision for the New England Orogen,eastern Australia

Structural, magnetic and gravity trends of the southern New England Orogen (SNEO) indicate four oroclinal structures, none conclusively confirmed paleomagnetically. Curved structures of the Tamworth Belt (TB)—a continental forearc exposed across six tectono-stratigraphic blocks with interlinked Carboniferous stratigraphies and extensive ignimbritic rocks known to retain primary magnetisations despite prevalent overprinting—are prospective to oroclinal testing through comparison of Carboniferous pole paths for individual blocks. Pole paths (a) have been established for the Rocky Creek and Werrie blocks (northwestern/western TB), (b) are described herein for the Rouchel Block (southwestern TB), and (c) are forthcoming for the Gresford and Myall blocks (southern/southeastern TB). The Rouchel path derives from detailed paleomagnetic, rock magnetic and magnetic fabric studies. Thermal, alternating field and liquid nitrogen demagnetisations show a low-temperature overprint, attributed to late Oligocene weathering, and high-temperature (HT) primary and overprint components in both magnetite and hematite carriers, showing slight, systematic, directional differences with hematite providing the better cleaned site poles. Seven primary mean-site poles of Tournaisian and mainly Visean age and three overprint poles show six positive fold tests, five at 95% or higher confidence levels. Two dispersed groupings of intermediate (IT) and HT overprint site poles of Permian and Permo-Triassic age are attributed to early and late phases in oroclinal evolution of the SNEO. HT and IT/HT overprint site poles of mid-Carboniferous age are attributed to Variscan Australia–Asia convergence. Individual pole paths for the Rocky Creek, Werrie and Rouchel blocks show no noticeable rotation between them, indicating primary curvature for the southwestern TB. Their integrated SNEO pole path establishes a reference frame for determining rotations of the southern and southeastern TB.     

滇西保山地块早古生代花岗岩类的年代学、地球化学及意义

位于滇西特提斯构造带保山地块的平河岩体,岩石类型主要为花岗岩、二长花岗岩,其次有少量的花岗闪长岩。4件样品的锆石U-Pb年龄变化于480～486Ma,表明这些花岗岩类侵位于早奥陶世。平河花岗岩类的K2O/Na2O值大于1,铝饱和指数(A/CNK)为1.07～1.1,属高钾钙碱性过铝质花岗岩。岩石总体上富集大离子亲石元素和Pb,亏损高场强元素。明显富集轻稀土元素[(La/Yb)N=4.33～7.05],显示明显的负Eu异常(δEu=0.25～0.48)。4件样品58个测点的锆石εHf(t)值变化范围较大(主要集中于-12.4～-3.0之间),对应的Hf同位素地壳模式年龄集中于2.2～1.7Ga。这些地球化学特征指示平河花岗岩类为S型花岗岩,主要来源于古老地壳物质(如砂屑岩)的重熔,并不同程度地混入了幔源物质。平河花岗岩类与出露于中部拉萨地体的变质酸性火山岩存在可比性,可能代表了早古生代冈瓦纳大陆原特提斯边缘岩浆弧的一部分。     

航空时间域电磁法在澳大利亚新英格兰造山带铜多金属矿靶区优选与评价中的应用

澳大利亚新英格兰造山带构造演化历史复杂,已知铜多金属矿床(点)众多,成矿条件较为优越,是实现铜多金属矿找矿突破的有利区域.通过以航空时间域电磁法测量为核心的工作手段,以航电、磁异常为基础,以"类比—求同"为靶区优选评价指导思想,对新英格兰造山带某探矿权内开展铜多金属矿靶区优选和评价工作.在该地形条件复杂的工作区内,快速、科学有效圈定并优选出铜多金属矿找矿靶区共计9处,其中Ⅰ类找矿靶区3处,Ⅱ类找矿靶区5处,Ⅲ类找矿靶区1处,为在境内外类似工作条件的地区快速有效开展铜多金属矿找矿靶区优选和评价工作提供了参考案例.