Crustal architecture of the Capricorn Orogen,Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.     

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.     

A geophysically constrained multi-scale litho-structural analysis of the Trans-Tanami Fault,Granites-Tanami Orogen,Western Australia

The Trans-Tanami Fault in the poorly exposed Paleoproterozoic Granites-Tanami Orogen of Western Australia is an ～100 km long curvilinear structure with ～6 km right lateral displacement. Multi-scale integration and analysis of aeromagnetic, gravimetric, reflection seismic and remote sensing data have constrained the relative timing and architectural relationship of this structure. Interpretation of regional scale long-wavelength potential field (gravity and magnetic) anomalies, which are commonly used to define first-order structures, show that the fault is not a terrane boundary. Structural interpretation of short-wavelength potential field data illustrates that the structural domains on either side of the fault represent the products of a non-homogeneous stress regime developed between rigid granitic plutons. Additionally, 2D joint forward modelling of gravity and magnetic data and interpretation of reflection seismic data confirms the vertical displacement across this fault to be negligible indicating a predominant lateral displacement. The lateral displacement along a portion of this structure has exploited a pre-existing plane of a north-dipping thrust fault. Where this early thrust fault terminates, the Trans-Tanami Fault displaces previously unfaulted rock as a wrench fault step-over. These observations differ from previous findings in the area by constraining the absolute displacement of this structure and through the recognition of a wrench fault system that includes lateral step-overs between re-activated early thrust fault planes.     

Beach sand of SE Australia traced by zircon ages through Ordovician turbidites and S-type granites of the Lachlan Orogen to Africa/Antarctica: a review

The Quaternary beach sand of SE Australia, driven northward by southern swell, contains zircons with dominant U–Pb ages of 700–500 Ma, model ages (T_DMc) of 2.2 Ga to 1.0 Ga, and ?_Hf of +12 to –30, indicating a host rock type of granitoids with alkaline affinity. These properties match those of detrital zircons in the Middle Triassic (ca 240 Ma) Hawkesbury Sandstone (T_DMc of 2.1 to 1.0 Ga, ?_Hf of +8 to –40, alkaline granitoids) and the Ordovician (ca 460 Ma) turbidites and ca 430 Ma S-type granitoids of the Lachlan Orogen (T_2DM of 2.0 to 1.0 Ga, ?_Hf of +5 to –30), all of which are identified as proximal provenances. Superimposed are the ca 400 Ma zircons in beaches in the south backed by the 420–375 Ma I-type Bega Batholith, and ca 350 Ma and ca 250 Ma zircons in the north backed by the New England Orogen. The Ordovician turbidites, part of a deep-sea super-fan, were fed by the detritus of the exhumed 700–500 Ma Transgondwanan Supermountains atop the East African–Antarctic Orogen. At the same time, the ancestral Gamburtsev Subglacial Mountains of East Antarctica probably contributed a subsidiary fan of 700–500 Ma sediment. Primary zircons aged 600–500 Ma in igneous and metamorphic rocks in Australia and the ancestral Transantarctic Mountains are minor contributors of the Australian sediments. The properties of the 700–500 Ma primary zircons in the East African–Antarctic Orogen are traceable through the first-cycle Ordovician turbidite and intruding second-cycle granite, and younger sediment, such as the third-cycle Triassic Hawkesbury Sandstone and the third-cycle beach sand. The sand at the northern terminus of the coastal system off Fraser Island spills over the shelf edge into the Tasman Abyssal Plain to reflect in miniature the deep-sea depositional environment of the Ordovician.     

Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

Abstract

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ¹⁸O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ?Hf_t range from ?8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ?Hf_t (～?7 to ?4) and elevated δ¹⁸O values (～9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ?Hf_t (～?9 to ?6) but more subdued δ¹⁸O values (～7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ?Hf and mantle-like δ¹⁸O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (?Hf_t as high as +8.5, δ¹⁸O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (?Hf_t as high as ?0.5, δ¹⁸O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two.     

皖南变质岩区古生物化石的发现和研究进展

皖南岩变岩分布区是古陆还是一个造山带？这一直是国内外地质学家讨论华南大地构造的焦点问题。在综合分析前人的研究及存在的问题后,提出以造山带的观点解体本区地层是未来发展的主要趋势;并以新发现的大量古生物化石为依据,讨论了目前地这个造山带研究取得的进展及需要进一步研究的问题,同时个华南的大地构造问题提供了新的资料。     

辽吉地区古元古代造山作用的大陆动力学过程及其壳内响应

古元古代是地质演化历史中一个重要转折时期,探讨其大陆动力学特征、机制、过程及其壳内响应具有重要的科学意义．本文以辽吉地区古元古代造山带为例,结合同位素年代学资料,综合分析了该区大陆动力学过程的构造、变质、岩浆、沉积等方面壳内响应特征最后,将该区深部大陆动力学过程划分为三大历史阶段：1)造山期前伸展阶段,包括建造伸展期和改造伸展期,深部底侵发生,壳内响应出现基底裂解、盖层顺层滑脱、拉张型垂向递增变质、岩浆呈席状侵位等;2)造山期挤压收缩阶段,俯冲碰撞,岩石田地幢可能出现拆沉,壳内响应出现招皱逆冲、双侧造山、侧向递增变质、碰撞型岩浆作用发生、早期岩浆底辟再就位等;3)造山期后伸展塌陷阶段,大陆岩石田在重力均衡作用下发生微弱伸展塌陷,出现环斑花岗岩侵位,等压冷却为特征的退变质表明没有明显的伸展剥蚀,没有出现明显的山根．     

Detrital zircon geochronology and geochemistry of metasediments from the Vorontsovka terrane: implications for microcontinent tectonics

ABSTRACT

The Vorontsovka terrane (VT) is an important component of the East Sarmatian Orogen (ESO) which divides the Precambrian cores of the Sarmatian and Volgo-Uralia segments of the East European Craton (EEC). The tectonic framework of the VT remains controversial due to poor constraints from geochemical and geochronological studies. In this article we present detrital zircon U–Pb ages and geochemical features of the Precambrian meta-sedimentary rocks from the VT, which occur interlayered with calc-silicate rocks and metabasites. Most of the zircons from metasediments possess oscillatory zoning and high Th/U ratios (>0.2), indicating magmatic provenance. Their ²⁰⁷Pb/²⁰⁶Pb ages cluster around 2093 ± 7, 2126 ± 7, 2158 ± 12, 2189 ± 16, and 2210 ± 31 Ma, correlating with the ages of magmatic zircon cores from the surrounding igneous suites, and reflecting a single tectono-magmatic cycle (~2200–2100 Ma) in the source area. Age of the youngest detrital zircon grain from the metasedimentary rocks and the cores of zircon grains from igneous suites show ²⁰⁷Pb/²⁰⁶Pb ages at 2094 and 2106 Ma, respectively. Together with the largest age clusters of 2126 ± 7 and 2158 ± 12 Ma of the magmatic cores of the detrital zircons, the timing of sedimentation is inferred as ~ 2100–2170 Ma.

The metapelites display strong rare earth element fractionation with variable Eu anomalies ((La/Yb)_N = 7.0–14.5, Eu/Eu* = 0.49–1.23). In contrast, the calc-silicate rocks and metabasites lack Eu anomalies ((La/Yb)_N = 5.2–11.5, Eu/Eu* = 0.87–1.00). The large-ion lithophile (LILE) and high field strength element (HFSE) concentrations of most samples are comparable with those of the upper continental crust (UCC). The rocks possess negative anomalies of Th, Nb, Sr, and Zr relative to UCC. Their high Index of Compositional Variability (0.85–1.32, up to 1.8 in metabasites) and relatively low Chemical Index of Alteration (46.1–70.4) indicate that the metapelitic sediments were immature to weakly immature and probably underwent minor chemical weathering. The protoliths of the metabasites are interpreted as interlayered volcano-sedimentary and pyroclastic material. Relict clastic textures of the VT rocks, their geochemical features, and the grain morphology of detrital zircons suggest that the sediments were derived from intermediate and felsic provenances, which were most likely deposited in an environment with active volcanism. We envisage an active continental margin setting in the southwestern part of the Volgo-Uralia segment of the EEC related to the assembly of the Palaeoproterozoic Columbia supercontinent. Combined with recent data from surrounding terranes of the ESO, our results suggest that the VT represents an accretionary prism along a continental arc within the Sarmatia and Volgo-Uralia oceanic realm in the Palaeoproterozoic.     

华北克拉通中部造山带早前寒武纪变质演化历史评述

根据变质作用程度不同,华北中部造山带早前寒武纪基底可以进一步分为高级区和花岗-绿岩带。前者变质程度可达高角闪岩相-麻粒岩相,包括太华、吕梁、阜平、恒山、怀安、宣化等杂岩,花岗-绿岩带的变质程度较低,多为绿片岩相-角闪岩相,包括登封、赞皇、五台等杂岩。已有变质演化研究表明,高级区恒山、怀安和宣化杂岩中的麻粒岩和(或)退变榴辉岩记录的峰期变质压力最高,恒山杂岩、阜平杂岩和宣化杂岩中的麻粒岩记录的峰期变质温度最高;花岗-绿岩带中的赞皇杂岩和五台杂岩出露高压斜长角闪岩和高压变泥质岩。中部带各变质杂岩中可识别出早期进变质、峰期、峰后快速降压和晚期冷却等变质阶段,拥有顺时针近等温降压型的变质作用P-T轨迹,与华北克拉通中部的俯冲碰撞有关。大量变质年代学数据显示,中部带各变质杂岩中至少记录了~1.85Ga、~1.95Ga和~2.5Ga三组变质年龄,其中,~1.85Ga的变质年龄占据了主导地位,大致与区域片麻理形成的时间一致,代表变质高峰期时代;~1.95Ga的变质年龄代表了峰期前的某个进变质片段;~2.5Ga的变质年龄则指示了更早一期的变质事件,推测与古老陆块~2.5Ga所遭受的大量幔源岩浆的侵入和底垫作用有关。然而,变质年龄与变质阶段的对应关系尚不明确。     

河南内乡县银洞沟银矿地质和流体包裹体特征及成因类型

河南内乡县银洞沟银(多金属)矿床位于东秦岭北部的二郎坪地体,属于断控脉状矿床。矿石有石英脉型和蚀变岩型,围岩蚀变以硅化、绢云母化、碳酸盐化、绿泥石化为主,主要银矿物为银黝铜矿、辉银矿、自然银等。成矿过程可以划分为早、中、晚3个阶段,分别形成含黄铁矿和/或毒砂的石英脉、多金属硫化物网脉和石英-碳酸盐细脉。脉石矿物石英、碳酸盐等含大量流体包裹体,包括富/纯CO2、含CO2水溶液和水溶液3种组分类型的包裹体。矿床形成于中温(270℃～370℃)、低盐度(<11.4%)、低密度流体系统。成矿早阶段以深源富碳流体为主,中阶段流体发生沸腾,晚阶段为盐水溶液。从早到晚,包裹体捕获压力由280MPa～320MPa降为90MPa～92MPa,成矿深度略有变浅,流体系统静岩压力变为静水压力体系,趋于开放,指示区域应力场由挤压向伸展转变。银洞沟银矿的地质特征和成矿流体系统与造山型金矿一致,应属造山型银矿;结合区域构造背景,认为银洞沟银矿形成于陆陆碰撞体制,并可以借助CMF模式解释成因。