云蒙山变质核杂岩抬升过程中伸展拆离和岩浆底辟联合作用的证据

北京地区云蒙山变质核杂岩在白垩纪阶段抬升的早期,伴随着沿四合堂剪切带由北向南的拆离滑脱和大型花岗闪长岩的垂向侵位,晚期变形发生在花岗闪长岩岩基周边及其邻近围岩中,形成云蒙山剪切带,并伴随大量同构造的花岗岩和伟晶岩岩脉灌入。剪切带中所有的岩脉都随时间发生了不同程度的变形,较老的岩脉形成紧闭的圆柱状褶皱,枢纽与剪切带的线理和面理接近平行。岩脉与剪切带中L-S组构的平行化作用主要是由于这种转动的结果。岩脉的成分和长英指数随它们的变形程度发生变化,说明持续的岩浆分异作用与韧性剪切变形是同时发生的。云蒙山剪切带由岩浆底辟引起的上盘岩石重力所驱动,并不断得到同构造侵位岩脉的补充,起到存储和不断改造侵位岩脉的作用。早期伸展体制下形成的四合堂剪切带局部遭受云蒙山剪切带的改造或复合。该地区的岩石、构造和同构造岩脉的变形几何学和运动学证据表明,太古宙结晶基底的抬升是下部岩浆底辟与上部地壳伸展拆离共同作用的结果。     

云蒙山变质核杂岩抬升过程中伸展拆离和岩浆底辟联合作用的证据

北京地区云蒙山变质核杂岩在白垩纪阶段抬升的早期,伴随着沿四合堂剪切带由北向南的拆离滑脱和大型花岗闪长岩的垂向侵位,晚期变形发生在花岗闪长岩岩基周边及其邻近围岩中,形成云蒙山剪切带,并伴随大量同构造的花岗岩和伟晶岩岩脉灌入。剪切带中所有的岩脉都随时间发生了不同程度的变形,较老的岩脉形成紧闭的圆柱状褶皱,枢纽与剪切带的线理和面理接近平行。岩脉与剪切带中L-S组构的平行化作用主要是由于这种转动的结果。岩脉的成分和长英指数随它们的变形程度发生变化,说明持续的岩浆分异作用与韧性剪切变形是同时发生的。云蒙山剪切带由岩浆底辟引起的上盘岩石重力所驱动,并不断得到同构造侵位岩脉的补充,起到存储和不断改造侵位岩脉的作用。早期伸展体制下形成的四合堂剪切带局部遭受云蒙山剪切带的改造或复合。该地区的岩石、构造和同构造岩脉的变形几何学和运动学证据表明,太古宙结晶基底的抬升是下部岩浆底辟与上部地壳伸展拆离共同作用的结果。     

辽西医巫闾山晚侏罗世伸展事件的时限来自同构造岩脉的年代学证据

位于燕山构造带东端的医巫闾山变质核杂岩经历了两次地壳伸展活动,早期在NNE-SSW拉伸背景下形成了现今围 绕医巫闾山岩体周缘分布的拆离韧性剪切带与相应的变质核杂岩,晚期隆升过程中由于拉伸方向转变又叠加发育了瓦子峪 伸展韧性剪切带。早期伸展变形中,围绕在拆离韧性剪切带及变形下盘中侵入了大量不同变形样式的同构造花岗岩脉。通 过对不同阶段侵入的岩脉进行LA-ICP-MS 锆石U-Pb 定年,显示该核杂岩的活动时间为157~149 Ma。医巫闾山地区晚侏罗 世伸展活动的确定,暗示华北克拉通北部可能从晚侏罗世已经开始发生破坏,到早白垩世达峰期并遍及整个华北克拉通东部。     

京北云蒙山地区四合堂韧性逆冲型剪切带的变形特征及早期变形时限约束

云蒙山杂岩是华北克拉通内记载了晚中生代构造演化的重要构造单元之一。云蒙山地区区域构造格架主要由四合堂背斜推覆体、四合堂逆冲型韧性剪切带、云蒙山背形、河防口正断层及水峪伸展型韧性剪切带等组成。对于四合堂逆冲型剪切带的活动时限及其与水峪剪切带之间的关系,是长期争论的课题之一。在四合堂韧性剪切带中广泛发育有剪切演化各个阶段就位的花岗质岩脉。本文研究云蒙山四合堂地区剪切变形特征及广泛发育的岩脉与构造变形之间关系,将岩脉厘定为构造期前(剪切前)岩脉、同构造期(同剪切)岩脉和构造期后(剪切后)岩脉等6期。不同类型的同构造岩脉锆石U-Pb定年获得了岩脉结晶年龄为170~150Ma。从构造-岩浆关系分析角度考虑,本文认为四合堂剪切带韧性逆冲作用早期的活动始于约170Ma,并持续到约150Ma。     

哀牢山-红河剪切带中段多阶段新生代花岗岩脉:同位素年代学及对于剪切应变型式转变时间的约束

哀牢山-红河剪切带是新生代印度板块与欧亚板块碰撞过程中发育的大规模走滑型剪切带,其发育对于碰撞过程中印支地块的南东向逃逸以及藏东南地区构造格局的形成具有重要的贡献。与剪切带演化相关,伴随发育多阶段花岗岩脉就位,它们为限定剪切变形时限、阐明剪切作用属性提供了重要证据。本文在野外观察基础上,应用显微构造和EBSD石英c-轴组构分析查明花岗岩脉的构造特点与应变型式,同时采用锆石LA-ICP-MS测年方法获得岩脉侵位与结晶年龄。年龄分析结果表明,岩脉年龄分别为27.09±0.48Ma、25.17±0.23Ma和25.16±0.50Ma,其中年龄为27.09±0.48Ma的花岗岩脉具有糜棱岩化现象,其变形特征体现为中温变形后叠加低温变形,且剪切变形形式由一般剪切转换为简单剪切;年龄为25.17±0.23Ma的花岗岩脉表现出同剪切晚期构造特征,且具有较低温度简单剪切变形特点;25.16±0.50Ma的切穿糜棱叶理,矿物未见变形,可能代表剪切期后岩脉。结合区域构造,推测剪切方式由纯剪为主的剪切向由单剪为主的剪切转换发生在27Ma和25Ma之间,哀牢山-红河剪切带中段在约25Ma走滑运动结束。     

华北克拉通中部带中段古元古代构造格架与主要变形事件研究

古元古代是华北克拉通构造演化的关键时期,中部带的性质是近年来华北克拉通构造演化研究争论的焦点之一。文中根据古元古代韧性剪切带重新划分了中部带北段构造格架,即由近EW向朱家坊韧性剪切带、NEE向王家会韧性剪切带和NE向龙泉关韧性剪切带分割恒山、繁峙、五台和阜平地块。这些地块基底具有不同的前寒武纪变质杂岩组成、构造变形特征和构造演化历史,一般都具多期变质变形叠加的特征。研究表明华北克拉通中部构造带北段在古元古代末发生3次重要的构造变形事件：繁峙地块由SW向NE方向的构造挤入,挤入过程中导致五台地块变形并向南东方向逆冲(D1);以阜平地块为核部的大规模伸展抬升,阜平变质杂岩抬升至近地表,阜平地块与五台地块之间形成一系列低角度伸展型韧性剪切带(D2);最后一次构造事件是NE向拉伸导致的大量NW向基性岩墙群侵位(D3)。     

华北中北部高级变质岩区的构造区划及其晚太古代构造演化

通过区域构造编图及重点地段的详细研究，在华北克拉通中北部识别出一条ＮＮＥ向的构造带——龙泉关－桑干带，它以大规模的韧性剪切带网络、重熔钾质花岗岩带、数量众多的高压麻粒岩构造透镜体或岩片为特征。剪切带以低角度逆冲为主，矿物拉伸线理指示运动方向为ＮＷ－ＳＥ向。这些剪切带造成东西两侧基底杂岩与表壳岩系包括孔兹岩系的广泛构造叠置，以及高压麻粒岩的近等温减压上隆。从更大的范围看，龙泉关－桑干带处于鄂尔多斯克拉通和阜平－赞皇克拉通之间，并且被五台－吕梁裂谷型绿岩带截切或不整合覆盖。该带应形成于晚太古代，记录了上述两个克拉通斜向拼合的构造过程     

脉岩对花岗岩体含金剪切带演化的控制作用─—以Val—d＇Or地区阿比提比绿岩带中Bourlamaque岩体为例

花岗岩体中与剪切带有关的含金石英脉与镁铁质脉岩密切相关，脉岩对剪切带的分布、方向及运动学机制都有着显著的控制作用。Ｖａｌ—ｄ＇Ｏｒ地区Ｂｏｕｒｌａｍａｑｕｅ岩休中出露若干条有经济意义的含金剪切带，其中大多数沿闪长质脉岩分布或叠加于闪长质脉岩之上。该区所有矿床都由赋存于斜冲断层内的石英—电气石—黄铁矿脉组成，这些脉的走向、倾向、滑移方向变化较大，矿脉的几何形态和运动学机制比在变形均一侵入体中所预想的简单共轭型式要复杂得多。Ｂｏｕｒｌａｍａｑｕｅ岩体外类似剪切带显示的北向性挤压与岩体内剪切带的应力张量一致，说明闪长质脉岩对岩体内复杂的剪切带和矿脉起了显著的控制作用。这些脉岩在后期变形中作为活化的薄弱层，进一步表明非均质薄弱层在剪切带形成演化过程中的重要作用。矿体的倾伏方向与矿体沿剪切带的滑移方向在空间上具有一定的几何关系：二者常常垂直或相互平行。因此勘探初期即可预测矿体的倾伏方向。由于矿脉沿剪切带活化层的滑移方向受应力大小及有关的薄弱层控制，因此利用充分资料，通过断层滑移分析方法，不仅可计算出控制参数，还可预测任一方向活化层的滑移方向。     

庐山变质核杂岩东侧拆离带两期构造性质转换:锆石U-Pb年代学证据

庐山变质核杂岩东侧的星子牛屎墩地区广泛岀露伸展拆离、韧性流变的构造现象,拆离方位为南东方向。该区还岀露一期NNE向左行走滑韧性剪切构造,推测是与郯庐断裂同期变形的构造产物,为郯庐断裂系的一部分。这两期构造运动反映了中生代太平洋构造体制下挤压应力向伸展应力的转换,对伸展滑脱层内同构造的伟晶岩脉及长英质脉的锆石U-Pb年代学测试,结合野外构造现象,以探究该区两期构造性质的转换时限和构造背景。新生变质流体结晶的锆石得到135~140Ma的庐山变质核杂岩拆离带的伸展年龄,内部受热液溶蚀作用的残余锆石得到150.5Ma和153.9Ma的左行剪切变形的年龄。受太平洋构造体制控制,晚侏罗世,该区受板块俯冲作用而处于挤压应力的构造背景,表现为左行剪切构造;早白垩世,在区域性的伸展、减薄作用下,挤压应力向伸展应力转换,庐山变质核杂岩得以形成,其伸展拆离构造是在早期左行剪切构造上的改造与叠加。     

新疆萨Ⅰ金矿床石英菱镁岩与金成矿作用研究

新疆西准噶尔萨Ⅰ金矿床由含金糜棱岩化石英菱镁岩和含金石英脉组成,矿体围岩为石英菱镁岩、碳酸盐-滑石片岩和蛇纹岩。根据岩相学研究,将萨Ⅰ金矿床的形成过程划分为5个阶段：黄铁矿-石英-碳酸盐阶段（Ⅰ）、自然金-辉砷镍矿-铬云母阶段（Ⅱ）、黄铁矿-碳酸盐-石英阶段（Ⅲ）、自然金-硫化物-石英阶段（Ⅳ）和碳酸盐阶段（Ⅴ）。阶段Ⅰ和阶段Ⅱ对应于石英菱镁岩经历的韧-脆性变形时期,其他3个阶段形成于脆性变形期间,反映出构造性质由挤压向拉伸环境转换的过程。阶段Ⅱ和阶段Ⅳ分别对应于Au-As和Au-Cu成矿作用。金矿化与石英菱镁岩经历的剪切变形过程相关。含金糜棱岩化石英菱镁岩的微量元素含量明显高于未变形的石英菱镁岩,说明剪切带流体输送了相关微量元素和成矿元素。在韧性变形向脆性变形转换过程中,流体压力骤降,含金络合物分解形成金矿。萨Ⅰ金矿床是产在糜棱岩化石英菱镁岩中的严格受剪切带控制的热液脉型金矿床。     

Tectonic evolution of the southern Gawler Craton,South Australia,from electromagnetic sounding

Long-period natural-source electromagnetic data have been recorded using portable three-component magnetometers at 39 sites in 1998 and 2002 across the southern Eyre Peninsula, South Australia that forms part of the Gawler Craton. Site spacing was of order 5 km, but reduced to 1 km or less near known geological boundaries, with a total survey length of approximately 50 km. A profile trending east – west was inverted for a 2D electrical resistivity model to a depth of 20 km across the southern Eyre Peninsula. The main features from the models are: (i) on the eastern side of the Gawler Craton, the Donington Suite granitoids to the east of the Kalinjala Shear Zone are resistive (>1000 Ωm); (ii) the boundary between the Donington Suite granitoids and the Archaean Sleaford Complex, which has much lower resistivity of 10 – 100 Ωm, is almost vertical in the top 10 km and dips slightly westwards; and (iii) two very low resistivity (<1 Ωm) arcuate zones in the top 3 km of Hutchison Group sediments correlate with banded iron-formations, and are probably related to biogenic-origin graphite deposits concentrated in fold hinges. Such features suggest an extensional regime during the time period 2.00 – 1.85 Ga. We suggest that the resistivity boundary between the Donington Suite and the Archaean Sleaford Complex represents a growth fault, typical for rift systems that evolve into a half-graben structure. In the graben basin, low-resistivity shallow-marine Hutchison Group sediments were deposited. Folding of the sediments during the Kimban Orogeny between 1.74 and 1.70 Ga has led to migration of graphite to the fold hinges resulting in linear zones of very low resistivity that correlate with banded iron-formation magnetic anomalies.     

Relationships between magmatism and deformation in northern Yorke Peninsula and southeastern Proterozoic Australia

The ca 1600–1580 Ma time interval is recognised as a significant period of magmatism, deformation and mineralisation throughout eastern Proterozoic Australia. Within the northern Yorke Peninsula in South Australia, this period was associated with the emplacement of multiple phases of the Tickera Granite, an intensely foliated quartz alkali-feldspar syenite, a leucotonalite and an alkali-feldspar granite. These granites belong to the broader Hiltaba Suite that was emplaced at shallow crustal levels throughout the Gawler Craton. Geochemical and isotopic analysis suggests these granite phases were derived from a heterogeneous source region. The syenite and alkali-feldspar granite were derived from similar source regions, likely the underlying ca 1850 Ma Donington Suite and/or the ca 1750 Ma Wallaroo Group metasediments with some contamination from an Archean basement. The leucotonalite is sourced from a similar but more mafic/lower crustal source. Phases of the Tickera Granite were emplaced synchronously with deformation that resulted in development of a prominent northeast-trending structural grain throughout the Yorke Peninsula region. This fabric is associated with composite events resulting from folding, shearing and faulting within the region. The intense deformation and intrusion of granites within this period resulted in mineralisation throughout the region, as seen in Wheal Hughes and Poona mines. The Yorke Peninsula shares a common geological history with the Curnamona Province, which was deformed during the ca 1600–1585 Ma Olarian Orogeny, and resulted in development of early isoclinal and recumbent folds overprinted by an upright fold generation, a dominant northeast-trending structural grain, mineralisation, and spatially and temporally related intrusions. This suggests correlation of parts of the Gawler Craton with the Curnamona Province, and that the Olarian Orogeny also affected the southeastern Gawler 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.     

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.     

Geology of the Acropolis prospect,South Australia,constrained by high-precision CA-TIMS ages

Abstract

Acropolis is an Fe-oxide–copper–gold prospect ～20?km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite?±?sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (ca 1850?Ma). A non-conformity spanning ca 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03?±?0.68?Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88?±?0.50?Ma) and felsic dykes (1593.88?±?0.56?Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was ca 1594?Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and ca 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.

1. KEY POINTS

2. Fe-oxide–apatite?±?sulfide veins are hosted by the Gawler Range Volcanics (1594.03?±?0.68?Ma), the Hiltaba Suite granite (1594.88?±?0.50?Ma) and Donington Suite granite (ca 1850?Ma).

3. The age of felsic dykes (1593.88?±?0.56?Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was ca 1594?Ma.

4. The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and ca 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

     

High-grade Paleoproterozoic reworking in the southeastern Gawler Craton,South Australia ∗

SHRIMP U–Pb geochronology and monazite EPMA chemical dating from the southeast Gawler Craton has constrained the timing of high-grade reworking of the Early Paleoproterozoic (ca 2450 Ma) Sleaford Complex during the Paleoproterozoic Kimban Orogeny. SHRIMP monazite geochronology from mylonitic and migmatitic high-strain zones that deform the ca 2450 Ma peraluminous granites indicates that they formed at 1725 ± 2 and 1721 ± 3 Ma. These are within error of EPMA monazite chemical ages of the same high-strain zones which range between 1736 and 1691 Ma. SHRIMP dating of titanite from peak metamorphic (1000 MPa at 730°C) mafic assemblages gives ages of 1712 ± 8 and 1708 ± 12 Ma. The post-peak evolution is constrained by partial to complete replacement of garnet–clinopyroxene-bearing mafic assemblages by hornblende–plagioclase symplectites, which record conditions of ～600 MPa at 700°C, implying a steeply decompressional exhumation path. The timing of Paleoproterozoic reworking corresponds to widespread deformation along the eastern margin of the Gawler Craton and the development of the Kalinjala Shear Zone.     

An apatite U-Pb thermal history map for the northern Gawler Craton,South Australia

Apatite U-Pb thermochronology was applied to granitoid basement samples across the northern Gawler Craton to unravel the Proterozoic, post-orogenic, cooling history and to examine the role of major fault zones during cooling. Our observations indicate that cooling following the ~2500 Ma Sleaford Orogeny and ~1700 Ma Kimban Orogeny is restricted to the Christie and Wilgena Domains of the central northern Gawler Craton. The northern Gawler Craton mainly records post-Hiltaba Event(~1590 Ma) U-Pb cooling ages. Cooling following the ~1560 Ma Kararan Orogeny is preserved within the Coober Pedy Ridge,Nawa Domain and along major shear zones within the south-western Fowler Domain. The Nawa Domain samples preserve U-Pb cooling ages that are 150 Ma younger than the samples within the Coober Pedy Ridge and Fowler Domain, indicating that later(~1300 Ma) fault movement within the Nawa Domain facilitated cooling of these samples, caused by arc collision in the Madura Province of eastern Western Australia. When compared to~(40)Ar/~(39) Ar from muscovite, biotite and hornblende, our new apatite U-Pb ages correlate well, particularly in regions of higher data density. Our data also preserve a progressive younging of U-Pb ages from the nucleus of the craton to the periphery with a stark contrast in U-Pb ages across major structures such as the Karari Shear Zone and the Southern Overthrust, which indicates the timing of reactivation of these major crustal structures. Although this interpolation was based solely on thermochronological data and did not take into account structural or other geological data, these maps are consistent with the structural architecture of the Gawler Craton and reveal the thermal footprint of known tectonic and magmatic events in the Gawler Craton.     

Interpretation of a ca. 1600–1580 Ma metamorphic core complex in the northern Gawler Craton,Australia

The Mount Woods Domain in the Gawler Craton, South Australia records a complex tectonic evolution spanning the Palaeoproterozoic and Mesoproterozoic. The regional structural architecture is interpreted to represent a partially preserved metamorphic core complex that developed during the ~1600–1580 Ma Hiltaba Event, making this one of the oldest known core complexes on Earth. The lower plate is preserved in the central Mount Woods Domain, which comprises the Mount Woods Metamorphics. These rocks yield a detrital zircon maximum depositional age of ~1860 Ma and were polydeformed and metamorphosed to upper amphibolite to granulite facies during the ~1740–1690 Ma Kimban Orogeny. The upper plate comprises a younger succession (the Skylark Metasediments) deposited at ~1750 Ma. Within the upper plate, sedimentary and volcanic successions of the Gawler Range Volcanics were deposited into half graben that evolved during brittle normal faulting. The Skylark Shear Zone represents the basal detachment fault separating the upper and lower plate of the core complex. The geometry of normal faults in the upper plate is consistent with NE-SW extension.Both the upper and lower plates are intruded by ~1795–1575 Ma Hiltaba Suite granitic and mafic plutons. The core complex was extensively modified during the ~1570–1540 Ma Kararan Orogeny. Exhumation of the western and eastern Mount Woods Domain is indicated by new ⁴⁰Ar/³⁹Ar biotite cooling ages that show that rock packages in the central Mount Woods Domain cooled past ~300 °C ± 50 °C at ~1560 Ma, which was ~20 million years before equivalent cooling in the western and eastern Mount Woods Domain. Exhumation was associated with activity along major syn-Kararan Orogeny faults.     

U–Pb zircon,zircon Hf and whole-rock Sm–Nd isotopic constraints on the evolution of Paleoproterozoic rocks in the northern Gawler Craton

U–Pb zircon analyses from a series of orthogneisses sampled in drill core in the northern Gawler Craton provide crystallisation ages at ca 1775–1750 Ma, which is an uncommon age in the Gawler Craton. Metamorphic zircon and monazite give ages of ca 1730–1710 Ma indicating that the igneous protoliths underwent metamorphism during the craton-wide Kimban Orogeny. Isotopic Hf zircon data show that 1780–1750 Ma zircons are somewhat evolved with initial ε_Hf values –4 to +0.9, and model ages of ca 2.3 to 2.2 Ga. Isotopic whole rock Sm–Nd values from most samples have relatively evolved initial ε_Nd values of –3.7 to –1.4. In contrast, a mafic unit from drill hole Middle Bore 1 has a juvenile isotopic signature with initial ε_Hf zircon values of ca +5.2 to +8.2, and initial ε_Nd values of ～+3.5 to +3.8. The presence of 1775–1750 Ma zircon forming magmatic rocks in the northern Gawler Craton provides a possible source for similarly aged detrital zircons in Paleoproterozoic basin systems of the Gawler Craton and adjacent Curnamona Province. Previous provenance studies on these Paleoproterozoic basins have appealed to the Arunta Region of the North Australian Craton to provide 1780–1750 Ma detrital zircons, and isotopically and geochemically similar basin fill. The orthogneisses in the northern Gawler Craton also match the source criteria and display geochemical similarities between coeval magmatism in the Arunta Region of the North Australian Craton, providing further support for paleogeographic reconstructions that link the Gawler Craton and North Australian Craton during the Paleoproterozoic.     

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.