Proterozoic – Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn Block and its tectonic implications

Aeromagnetic and field data suggest that meta‐igneous rocks exposed on the south coast of central Victoria at Waratah Bay, Phillip Island, Barrabool Hills and inland near Licola, are continuous—beneath Bass Strait—with Proterozoic/Cambrian igneous rocks in King Island and Tasmania. This correlation is supported by a pre‐Early Ordovician unconformity above gabbro protomylonite at Waratah Bay, age equivalent to the Tasmanian Tyennan unconformity. Cambrian volcanics at Licola and unusual features of the Melbourne Zone sequence indicate that Tyennan continental crust extends north as basement to the central Victorian portion of the Lachlan Fold Belt. In contrast, adjacent parts of the Lachlan Fold Belt in Victoria contain conformable sea‐floor sequences that span the Early Cambrian to Late Ordovician, with no evidence of either Cambrian deformation or underlying continental basement. The block of Tyennan continental crust beneath central Victoria—the Selwyn Block—is fundamentally different, and has influenced temporal and spatial patterns of sedimentation, deformation, metamorphism and plutonism. Palaeogeographical reconstructions suggest that the block was a submarine plateau that lay outboard of the Australian craton, upon which a condensed Ordovician sequence was deposited. The sequence above the Selwyn Block unconformity at Waratah Bay is similar to widespread post‐Tyennan sediments in western Tasmania. During Late Ordovician and Early Silurian deformation, the Selwyn Block protected much of the overlying sedimentary sequence. Instead, shortening was focused into the Stawell and Bendigo Zones to the west. These zones were sandwiched between the Selwyn Block and the Australian craton in a ‘vice’ scenario reminiscent of some Appalachian orogenic events. The region above the Selwyn Block was downwarped adjacent to the overthrust Bendigo Zone as a foreland deep, into which a conformable clastic wedge of sediment was deposited in Late Ordovician to Devonian time, prior to final Middle Devonian deformation. The Selwyn Block includes the Cambrian calc‐alkaline Licola and Jamieson Volcanics that are correlated with the Tasmanian Mt Read Volcanics. In Victoria, these form a basement high controlling the unusual down‐cutting thrusts in the overlying Melbourne Zone and explaining the major structural vergence reversal between the Melbourne and Tabberabbera Zones. The Selwyn Block has exerted some control on the timing, chemistry and distribution of post‐orogenic granites, and on central Victorian gold mineralisation. Reactivated faults in the block influenced deposition, and continue to control the deformation of the portions of the Otway and Gippsland Basins that lie above it.     

Lamprophyric intrusions of probable carbonatitic affinity from South Australia

Primary carbonate‐bearing lamprophyric rocks have been recognised in the Walloway Diapir, South Australia. Their petrography and trace‐element geochemistry indicates that they have carbonatitic affinities. The rocks are magnetic and can be detected by ground magnetometer surveys.     

Reconstruction of the early Proterozoic stratigraphy of the Gawler Craton,South Australia

Detailed structural mapping on NE Eyre Peninsula, South Australia, has led to a revised stratigraphy and model of sedimentation for Early Proterozoic metasediments of the Gawler Craton. Four stages of deformation have been recognised; three stages are associated with the Kimban Orogeny (c. 1820–1580 Ma) and a fourth stage is known as the Wartakan Event (c. 1500–1450 Ma). The recognition of major D₂ folds has shown the previously used stratigraphy to be incorrect and has necessitated its revision. At the base of the sequence, unconformably overlying a 2300 Ma or older basement, is the Warrow Quartzite. A transgressive cycle of schist, dolomite (Katunga Dolomite) and iron formation (Lower Middleback Jaspilite) overlies the quartzite, and this is overlain in turn by a regressive semipelitic unit containing local amphibolites (Cook Gap Schist), and another transgressive iron‐formation bearing cycle (Upper Middleback Jaspilite). At the top of the sequence is the Yadnarie Schist. All units overlying the older basement to the top of the Yadnarie Schist are defined collectively as the Hutchison Group. The Middle‐back ‘Group’ consisting of units from the top of the Warrow Quartzite to the base of the Yadnarie Schist is redefined as the Middleback Subgroup. Sediments of the Hutchison Group were probably derived from 2300+ Ma rocks on western Eyre Peninsula and deposited on a shallow platform now oriented approximately N‐S.     

Continental ca 1.7 – 1.69 Ga Fe-rich metatholeiites in the Curnamona Province,Australia: a record of melting of a heterogeneous,subduction-modified lithospheric mantle

Mg-rich and Fe-rich metatholeiites intruded the Willyama Supergroup of the southern Australian Curnamona Province in the Late Palaeoproterozoic at ca 1700 Ma and 1685 Ma, respectively. Intrusion of the Fe-rich metatholeiites occurred during a period of punctuated extension in the Willyama basin. Major-element concentrations are variable (SiO₂ 45.4 – 56.5 wt%; Fe₂O₃? 8.5 – 20.7; TiO₂ 0.46 – 2.52 wt%; Mg# 70.5 – 29.1) and, in conjunction with trace-element data, support near-closed-system fractionation of a mantle-derived melt with little or no replenishment. Fractionation produced progressively Fe-rich derivative melts. Crystallising phases were dominated by clinopyroxene and olivine, whereas Fe – (Ti) oxide crystallisation was hindered. Primitive mantle-normalised immobile trace elements are characterised by variable Th, Nb, Sr, P and Ti anomalies. Chondrite-normalised rare-earth element patterns for the most primitive, Mg-rich samples from the western Broken Hill Domain have La_N/Sm_N < 1, whereas the most evolved Fe-rich samples from the Olary Domain have ratios of La_N/Sm_N > 1. Initial εNd values range between – 2.2 and + 2.7 for the majority of the samples, with the isotopic compositions showing no correlation with differentiation or assimilation. The combined geochemical and isotopic data suggest that the southern Curnamona Province metatholeiites were extracted from a depleted mantle in the western Broken Hill Domain, and a variably enriched, heterogeneous subcontinental lithospheric mantle in the Olary Domain. Magmatism most likely occurred in a backarc basin or intracontinental setting. It is speculated that the geochemically enriched mantle component was derived from subduction-related processes, probably related to pre-Willyama basin accretionary processes along the southern and eastern margins of the North Australian Craton.     

Temporal constraints on the timing of high-grade metamorphism in the northern Gawler Craton: implications for assembly of the Australian Proterozoic

LA-ICPMS U–Pb data from metamorphic monazite in upper amphibolite and granulite-grade metasedimentary rocks indicate that the Nawa Domain of the northern Gawler Craton in southern Australia underwent multiple high-grade metamorphic events in the Late Paleoproterozoic and Early Mesoproterozoic. Five of the six samples investigated here record metamorphic monazite growth during the period 1730–1690 Ma, coincident with the Kimban Orogeny, which shaped the crustal architecture of the southeastern Gawler Craton. Combined with existing detrital zircon U–Pb data, the metamorphic monazite ages constrain deposition of the northern Gawler metasedimentary protoliths to the interval ca 1750–1720 Ma. The new age data highlight the craton-wide nature of the 1730–1690 Ma Kimban Orogeny in the Gawler Craton. In the Mabel Creek Ridge region of the Nawa Domain, rocks metamorphosed during the Kimban Orogeny were reworked during the Kararan Orogeny (1570–1555 Ma). The obtained Kararan Orogeny monazite ages are within uncertainty of ca 1590–1575 Ma zircon U–Pb metamorphic ages from the Mt Woods Domain in the central-eastern Gawler Craton, which indicate that high-grade metamorphism and associated deformation were coeval with the craton-scale Hiltaba magmatic event. The timing of this deformation, and the implied compressional vector, is similar to the latter stages of the Olarian Orogeny in the adjacent Curnamona Province and appears to be part of a westward migration in the timing of deformation and metamorphism in the southern Australian Proterozoic over the interval 1600–1545 Ma. This pattern of westward-shifting tectonism is defined by the Olarian Orogeny (1600–1585 Ma, Curnamona Province), Mt Woods deformation (1590–1575 Ma), Mabel Creek Ridge deformation (1570–1555 Ma, Kararan Orogeny) and Fowler Domain deformation (1555–1545 Ma, Kararan Orogeny). This westward migration of deformation suggests the existence of a large evolving tectonic system that encompassed the emplacement of the voluminous Hiltaba Suite and associated volcanic and mineral systems.     

Timing of deformation and exhumation across the Karari Shear Zone,north-western Gawler Craton,South Australia

In the north-western Gawler Craton of South Australia, the Karari Shear Zone defines a boundary between late-Archean to earliest Paleoproterozoic rocks, which have remained largely undisturbed since the earliest Paleoproterozoic, and younger Paleoproterozoic rocks that have been reworked through multiple late Paleoproterozoic and Mesoproterozoic metamorphic and deformation events. The history of movement across the Karari Shear Zone has been investigated via new U–Pb and ⁴⁰Ar/³⁹Ar geochronology, in combination with pre-existing geochronological and metamorphic constraints, as well as the structural geometry revealed by a recently acquired reflection seismic transect. The available data suggest a complex history of shear-zone movement in at least four stages, with contrasting sense of motion at different times. The first period of movement across the Karari Shear Zone is inferred to have been a period of extension at ca 1750–1720 Ma. This was likely closely followed by reactivation during the Kimban Orogeny between ca 1720 and 1680 Ma, although the sense of movement during this period is unclear. Further reactivation, in a thrust sense, occurred between ca 1580 and 1560 Ma, resulting in significant exhumation of marginal domains of the Gawler Craton to the north of the Karari Shear Zone. A final episode of largely strike-slip shear-zone movement occurred at ca 1450 Ma.     

Timing and style of high-temperature metamorphism across the Western Gawler Craton during the Paleo- to Mesoproterozoic

Abstract

Combined in situ monazite dating, mineral equilibria modelling and zircon U–Pb detrital zircon analysis provide insight into the pressure–temperature–time (P–T–t) evolution of the western Gawler Craton. In the Nawa Domain, pelitic and quartzo-feldspathic gneisses were deposited after ca 1760?Ma and record high-grade metamorphic conditions of ～7.5?kbar and 850?°C at ca 1730?Ma. Post-peak microstructures, including partial plagioclase coronae and late biotite around garnet, and subtle retrograde garnet compositional zoning, suggest that these rocks cooled along a shallow down-pressure trajectory across an elevated dry solidus. In the northwest Fowler Domain (Colona Block), monazite grains from pelitic gneisses record two stages of growth/recrystallisation interpreted to represent discrete parts of the P–T path: (1) ca 1710?Ma monazite growth during prograde to peak conditions, and (2) ca 1690?Ma Y-enriched monazite growth/recrystallisation during partial garnet breakdown and cooling towards the solidus. Relict prograde growth zoning in garnet suggests rocks underwent a steep up-P path to peak conditions of ～8?kbar at 800?°C. The new P–T–t results suggest basement rocks of the southwestern Nawa and northwestern Fowler were buried to depths of 20–25?km during the Kimban Orogeny, ca 10 Myrs after the sedimentary precursors were deposited. The P–T path for the Kimban Orogeny is broadly anti-clockwise, suggesting that at least the early phase of this event was associated with extension. Exhumation of rocks from both the southwestern Nawa and northwestern Fowler domains may have occurred during the waning stages of the Kimban Orogeny (<ca 1690?Ma). The limited low-grade overprint in these rocks may be explained by a mid-to-upper crustal position for these rocks during the subsequent Kararan Orogeny. Aluminous quartz-feldspathic gneiss of the Nundroo Block in the eastern Fowler Domain records peak conditions of ～7?kbar at 800?°C. Monazite grains from the Nundroo Block are dominated by an age peak at ca 1590?Ma, although the presence of some older ages up to ca 1690?Ma, possibly reflect partial resetting of older monazite domains. The P–T–t conditions suggest these rocks were buried to 20–25?km at ca 1590?Ma during the Kararan Orogeny. This high-grade metamorphism in the Nundroo Block is a mid-crustal expression of the same thermal anomaly that caused magmatism in the central-eastern Gawler Craton. Juxtaposition of rocks affected by the Kimban and Kararan orogenic events in the western Gawler Craton was controlled by lithospheric-scale shear zones, some of which have facilitated ～20 kilometres of exhumation.     

基于PSO和LM算法的坐标转换新方法

3维坐标的转换精度与坐标转换参数的解算精度密切相关,在不同区域的坐标转换参数不完全相同,为了提高测量的精度,就必须求出适合本地区的坐标转换参数,以提高坐标转换的精度。本文直接从坐标转换的非线性方程出发,根据最优化问题的极值条件,研究采用基于微粒群优化(PSO)和拉凡格氏(LM)组合算法求解3维坐标转换参数。结果表明,该方法具有简单性、高效性和普适性,适合测量中3维坐标的转换解算。     

Interpreting prograde-growth histories of Al2SiO5 triple-point rocks using oxygen-isotope thermometry: an example from the Truchas Mountains, USA

Oxygen‐isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al₂SiO₅ (AS) polymorph‐invariant point as previously suggested. Instead, oxygen‐isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz‐fibrolite oxygen‐isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen‐isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen‐isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain δ¹⁸O values of crystallization make these minerals ideal to model prograde‐growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events.     

陕西秦岭元古宇松树沟岩组变质火山岩的地球化学特征及地质意义

中新元古界松树沟岩组出露于周至黑河黄草坡南及榨水丰河街北等地,为一套呈残块分布的含气孔、杏仁,局部具枕状构造,角闪岩相变质的中基性火山岩系夹少量碎屑岩。通过对该岩组的分析,其与洋中脊岩石化学特征较为相似,曲线较为平坦,轻稀土元素相对富集、重稀土元素相对亏损。微量元素蛛网图显示高场强元素及大离子亲石元素富集的特征。从构造环境进行分析,松树沟岩组形成于洋中脊环境。