首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Structural and thermochronological studies of the Kampa Dome provide constraints on timing and mechanisms of gneiss dome formation in southern Tibet. The core of Kampa Dome contains the Kampa Granite, a Cambrian orthogneiss that was deformed under high temperature (sub-solidus) conditions during Himalayan orogenesis. The Kampa Granite is intruded by syn-tectonic leucogranite dikes and sills of probable Oligocene to Miocene age. Overlying Paleozoic to Mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite + staurolite grade at the base of the sequence to unmetamorphosed at the top. The Kampa Shear Zone traverses the Kampa Granite — metasediment contact and contains evidence for high-temperature to low-temperature ductile deformation and brittle faulting. The shear zone is interpreted to represent an exhumed portion of the South Tibetan Detachment System. Biotite and muscovite 40Ar/39Ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14.22 ± 0.18 to 15.54 ± 0.39 Ma cooling ages and concordant spectra with 14.64 ± 0.15 to 14.68 ± 0.07 Ma cooling ages. Petrographic investigations suggest disturbed samples are associated with excess argon, intracrystalline deformation, mineral and fluid inclusions and/or chloritization that led to variations in argon systematics. We conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at  14.6 Ma. The Kampa Granite yields the youngest biotite 40Ar/39Ar ages of  13.7 Ma immediately below the granite–metasediment contact. We suggest that this age variation reflects either varying mica closure temperatures, re-heating of the Kampa Granite biotites above closure temperatures between 14.6 Ma and 13.7 Ma, or juxtaposition of rocks with different thermal histories. Our data do not corroborate the “inverse” mica cooling gradient observed in adjacent North Himalayan gneiss domes. Instead, we infer that mica cooling occurred in response to exhumation and conduction related to top-to-north normal faulting in the overlying sequence, top-to-south thrusting at depth, and coeval surface denudation.  相似文献   

2.
造山带穹隆构造记录了陆-陆碰撞及其碰撞后地壳和地表演化过程的信息,是探讨造山带构造演化的重要窗口。康巴穹隆位于藏南特提斯喜马拉雅地区,是北喜马拉雅片麻岩穹隆带(NHGD)的组成部分,其剥露过程及其动力学机制仍然存在争议。通过对康巴穹隆核部花岗片麻岩开展锆石U-Pb、锆石裂变径迹(ZFT)年代学研究和三维数值模拟,获得了康巴穹隆的锆石U-Pb年龄为497.89±1.2Ma,锆石FT年龄(17~11 Ma)明显小于锆石U-Pb结晶年龄,说明这些径迹年龄是岩体冷却抬升形成的。Pecube三维数值模拟对穹窿核部样品的ZFT数据进行反演显示,康巴穹隆核部岩体自中新世以来经历15.9~11.4Ma和ca. 4.2Ma两次快速剥露,结合区域构造演化,提出第一次快速剥露与藏南拆离系(STDS)的活动有关,第二次快速剥露是对气候变化过程的响应。  相似文献   

3.
Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D1, was preceded by an extensional event D1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D2, a major shortening event, which folded this sequence. D2 must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D3–D4, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D2–D4 lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D2–D4 deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D2 folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding.  相似文献   

4.
Amphibolite-grade quartzofeldspathic gneiss domes surrounded by greenschist-grade island arc and ophiolitic assemblages is a characteristic feature of the Arabian–Nubian Shield in the Eastern Desert of Egypt. The mode of formation of these domes, including the Meatiq Gneiss Dome, is controversial, as is the protolith age of these gneisses. Reinvestigation of selected segments of the Eastern Desert Shear Zone (EDSZ), a high-strain zone separating the eugeoclinal units from the underlying quartzofeldspathic gneisses show it to be a top-to-the NW shear zone which was later folded about a NW–SE trending fold axis (long axis of the gneiss dome). Kinematic indicators (shear bands, duplex structures, etc.) along the north-eastern and south-western flanks of the dome therefore show apparent left-lateral and right-lateral strike-slip displacement across the EDSZ. These observations are in conflict with most previous tectonic models which link formation of the dome to extension in a NW–SE oriented corridor bordered by two sub-parallel left-lateral NW–SE oriented strike-slip faults. Emplacement of upper crustal, low-grade, eugeoclinal rocks tectonically on top of middle crustal amphibolite-grade quartzofeldspathic gneisses indicates that the EDSZ may represents an extensional fault with a possible break-away zone in the southern part of the Eastern Desert. Alternatively it can be explained as the result of two (or more) tectonometamorphic events with an intervening episode of erosion and exhumation of high grade rocks prior to emplacement of the eugeoclinal thrust complex. Recent U–Pb TIMS ages on syntectonic orthogneisses and post-tectonic granites in the area show that shearing and subsequent doming must be younger than 630 Ma, possibly as young as 600 Ma.  相似文献   

5.
The Montagne Noire in the southernmost French Massif Central is made of an ENE‐elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low‐grade petrology, K–Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward‐directed Variscan nappe‐thrusting (D1) and a related medium‐P metamorphism (M1) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono‐metamorphic event D2/M2 largely transposes D1 structures, comprises a higher metamorphic thermal gradient than M1 (transition low‐P and medium‐P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post‐dating D1/M1 by more than 20 Ma. D2‐related fabrics are refolded by D3, which in its turn, is followed by dextral‐normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faugères units, D2/M2 is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D2/M2 also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low‐P/high‐T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D2 fabrics and the M2 metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone–epizone boundary to the diagenetic zone. The related zonation of the pre‐D3 metamorphic field gradients paralleled the dome. These observations indicate that D2/M2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D2/M2. Based on integration of these findings with regional geological constraints, a two‐stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull‐apart‐like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north‐dipping detachments and hangingwall basin formation along the northeastern dome margin.  相似文献   

6.
The Rössing granite-hosted uranium deposit in the Central Zone of the Pan-African Damara Orogen, Namibia, is situated in the “SJ area” to the south of the Rössing Dome. The coincidence of a number of features in this area suggests that mineralization is closely linked to late-kinematic evolution of the Rössing Dome. These features include: (1) the rotation of the dome's long axis (trend of 017°), relative to the regional F3 trend of 042°; (2) southward dome impingement, concomitant with dome rotation, producing a wedge-shaped zone of alkali-leucogranites, within which uranium mineralization is transgressive with respect to granites and their host lithologies; uranium mineralization and a high fluid flux are also confined to this arcuate zone to the south and south-east of the dome core and (3) fault modeling that indicates that the SJ area underwent late-D3 to D4 brittle–ductile deformation, producing a dense fault network that was exploited by leucogranites. Dome rotation and southward impingement occurred after a protracted period of transtensional tectonism in the Central Zone, from ca. 542 to 526 Ma, during which I- and S-type granites were initiated in a metamorphic core complex. Late-kinematic deformation involved a rejuvenation of the stresses that acted from ca. 600 to 550 Ma. This deformation overlapped with uranium-enriched granite intrusion in the Central Zone at 510 ± 3 Ma. Such late-kinematic, north–south transpression, which persisted into the post-kinematic cooling phase until at least 478 ± 4 Ma, was synchronous with left-lateral displacement along NNE-trending (“Welwitschia Trend”) shears in the vicinity of Rössing. Late-kinematic deformation, causing block rotation, overlying dome rotation and interaction of the more competent units of the Khan Formation with the Rössing Formation in the dome rim was pivotal in the localization of uranium-enriched granites within a highly fractured, high-strain zone that was also the site of prolonged/high fluid flux.  相似文献   

7.
A low-angle thrust fault places high-PT granulites (hangingwall) of the Internal Zone of the Neoproterozoic Brasília Belt (Tocantins Province, central Brazil) in contact with a lower-grade footwall (External Zone) comprised of nappes of distal passive margin- and back-arc basin-related supracrustals. The footwall units were emplaced at  750 Ma onto proximal sedimentary rocks (Paranoá Group) of the São Francisco paleo-continent passive margin. The high-PT belt is comprised of 645–630 Ma granulite-facies paragneiss and orthogneiss, and mafic–ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at  750 Ma. These complexes occur within lower-grade metasedimentary rocks in the hangingwall of the Maranhão River Thrust, which forms the Internal Zone–External Zone boundary fault to the north of the Pirineus Zone of High Strain. Detailed lithostructural studies carried out in Maranhão River Thrust hangingwall and footwall metasedimentary rocks between the Niquelândia and Barro Alto complexes, and also to the east of these, indicate the same lithotypes and Sm–Nd isotopic signatures, and the same D1–D2 progressive deformation and greenschist-facies metamorphism. Additionally, footwall metasedimentary rocks exclusively display a post-D2 deformation indicating that the Maranhão River Thrust propagated through upper crustal rocks of the Paranoá Group relatively late during the tectonic evolution of the belt. Fault propagation was a consequence of intraplate underthrusting during granulite exhumation. The results allow for a better tectonic understanding of the Brasília Belt and the Tocantins Province, as well as explaining the presence of the Pirineus Zone of High Strain.  相似文献   

8.
Knowledge of the Cretaceous–Tertiary history of upper crustal shortening and magmatism in Tibet is fundamental to placing constraints on when and how the Tibetan plateau formed. In the Lhasa terrane of southern Tibet, the widely exposed angular unconformity beneath uppermost Cretaceous–lower Tertiary volcanic-bearing strata of the Linzizong Formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs. during the Indo-Asian collision. In the Linzhou area, located  30 km north of the city of Lhasa, a > 3-km-thick section of the Linzizong Formation lies unconformably on Cretaceous and older rocks that were shortened by both northward- and southward-verging structures during the Late Cretaceous. The Linzizong Formation dips northward in the footwall of a north-dipping thrust system that involves Triassic–Jurassic strata and a granite intrusion in the hanging wall. U–Pb zircon geochronologic studies show that the Linzizong Formation ranges in age from 69 Ma to at least 47 Ma and that the hanging wall granite intrusion crystallized at  52 Ma, coeval with dike emplacement into footwall Cretaceous strata. 40Ar/39Ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 Ma, followed by an episode of accelerated cooling to upper crustal levels beginning at  42 Ma. The onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern Tibet and is interpreted be a consequence of either (1) Tertiary thrusting in this region or (2) regional rock uplift and erosion following removal of overthickened Gangdese arc lower crust and upper mantle or break-off of the Neo-Tethyan oceanic slab.  相似文献   

9.
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.  相似文献   

10.
In the Wadi Um Had area, Central Eastern Desert, Egypt, NE-trending metapelitic and molasse-type successions are exposed. The metasediments bear the geochemical signature of a first depositional cycle in two distinct continental island arc settings that derived from incipiently-to moderately-weathered intermediate to felsic sources under generally warm and humid conditions. The metapelitic succession records three distinct episodes of metamorphism, M1–M3, whereas the molasse-type succession records only the last metamorphic episode, M3. M1/D1 records an amphibolite facies tectono-metamorphic event that has been dated at 625 ± 5 Ma, whereas M2/D2 records a greenschist facies subduction-related event. Collision of the two domains during a NE–SW shortening D3, culminated in formation of the macroscopic NW–SE-trending folds. D2 and D3 correlate with the gneiss-forming event, which is constrained at <609 Ma, and doming of the nearby Meatiq gneiss dome, respectively. M3 is a hornblende hornfels facies thermal metamorphism related to the intrusion of the post-orogenic, Neoproterozoic (596.3 Ma) Um Had granite. This study records, for the first time, a tectono-metamorphic phase predating the gneiss-forming event in the Meatiq gneiss dome, and pushes the boundary of the Late Ediacaran terminal collision between East and West Gondwana to ≤600 Ma.  相似文献   

11.
张进江  杨雄英  戚国伟  王德朝 《岩石学报》2011,27(12):3535-3544
本文报道吉隆北喜马拉雅地区马拉山穹窿核部浅色花岗岩的锆石SHRIMP U-Pb和白云母激光40Ar/39 Ar年代学研究.花岗岩U-Pb年龄显示,穹窿核部浅色花岗岩岩浆活动(深熔及侵位)发生于～30Ma至～17Ma,其中最年轻的U-Pb年龄(17Ma)以及花岗岩白云母40Ar/39Ar年龄(17～15Ma)指示了马拉山穹窿的最后岩浆侵位时间及可能的穹窿冷却事件.已有研究表明,北喜马拉雅片麻岩穹窿带(NHGD)与藏南拆离系(STDS)中浅色花岗岩具有相似的最早侵位年龄,即～35 Ma,而STDS下盘U-Pb年龄老于35Ma的浅色花岗岩为增厚地壳重熔成因,表明北喜马拉雅在 ～35Ma地壳构造体制由挤压转为伸展,并暗示在始新世-渐新世转换期可能存在一更广泛意义的地质事件.～35 Ma以前增厚导致中下地壳部分熔融,形成中下地壳渠道流,渠道流活动触发增厚造山楔的垮塌,形成STDS.STDS的伸展减薄引发更大规模浅色花岗岩侵位,花岗岩底辟作用形成了NHGD,本文最年轻U-Pb年龄及40Ar/39Ar年龄(17～15Ma)即代表马拉山的底辟与穹窿作用,之后的构造体制由东西向伸展所取代(始于～13Ma).  相似文献   

12.
The geologic framework of the Phanerozoic Qinling–Dabie orogen was built up through two major suturing events of three blocks. From north to south these include the North China craton (including the north Qinling block), the Qinling–Dabie microblock, and the South China craton (including the Bikou block), separated by the Shangdan and Mianlue sutures. The Mianlue suture zone contains evidence for Mesozoic extrusion tectonics in the form of major strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200 Ma thrust belt that reformed by 200–150 Ma thrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian to Early Triassic metasandstones and metapelites, forming a regional-scale block-in-matrix mélange fabric. Five distinct tectonic units have been recognized in the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices including Early Paleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing rift margin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentary rocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separate basin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinct deformation episodes (D1, D2 and D3 during 240–200 Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the D1 episode. Thrust slices 20 km south of the Mianlue suture are related to this vertical extrusion due to the same rock assemblages, ages and kinematics. The D2 and D3 episodes folded all the units in a thick-skinned style about east–west (D2) and west–northwest (D3) axes in the Mianlue suture zone. An early foreland propagating sequence of accretion of Late Paleozoic rocks deposited above the Yangtze craton is not involved in D1 deformation but is temporally equivalent to the D2 and D3 deformation in the Mianlue suture. Two stages of strike–slip faulting mainly occurred at the end of D2 and D3, respectively. During D2 deformation, the Bikou block was obliquely indented to the ESE into the Mianlue suture, rather than being thrust over the Mianlue suture from the north as a part of the Qinling–Dabie microblock. During D3 deformation, however, the Bikou block was bounded by the south boundary fault of the Mianlue suture, and the Yangpingguan fault on the south. These faults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganze orogen. The other basement blocks north of the Mianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of the Wudang dome, during the D3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D3 emplacement of granite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D3 deformation. Therefore, based on insights from the evolution of the Mianlue suture, the D2 and D3 episodes in the Mianlue suture and its neighbors are not responsible for and associated with the two-stage extrusion of the Dabie UHP-HP terranes from the Foping dome to the present erosional surface (more than 350 km).  相似文献   

13.
Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.  相似文献   

14.
The metamorphic evolution of the Garzón Massif, Colombia, is established on the basis of the textural, goethermobarometric, and geochronological relationships of the metamorphic minerals. The geothermobarometric data define a clockwise, nearly isothermal decompression path (ITD) for rocks from Las Margaritas migmatites, constrained by four P–T areas: 780–826 °C and 6.3–8.0 kbar, 760–820 °C and 8.0–8.8 kbar, 680–755 °C and 6.6–9.0 kbar, and 630 °C and 4 kbar. For the a garnet-bearing charnockitic gneiss from the Vergel granulites, the path is counterclockwise, constrained by geothermobarometric data of 5.3–6.2 kbar and 700–780 °C and 6.2–7.2 kbar and 685–740 °C. The clockwise ITD path represents a loop followed by the orogen during the transitional granulite–amphibolite metamorphic conditions, probably associated with a subduction process followed by a collisional tectonic event. This subduction framework produced continental crust thickening between 1148 and 1034 Ma and later collision with another continental block approximately 1000 Ma ago. The orogenic exhumation occurred with moderate uplift rate. The counterclockwise trajectory and two metamorphic events suggest a vertical displacement between the Vergel granulites and Las Margaritas migmatites units, because there is no isotopic difference that indicates the existence of different terranes. The data confirm that the metamorphic evolution for this domain was more dynamic than previously believed and includes: (1) metamorphic processes with the generation of new crust with a possible mixture of old material and (2) metamorphic recycling of continental crust. These geological processes characterize a complex Mesoproterozoic orogenic event that shares certain features with the Grenvillian basement rocks participating in the formation of Rodinia.  相似文献   

15.
The Permian–Jurassic Mahanadi and Pranhita–Godavari Rifts are part of a drainage system that radiated from the Gamburtsev Subglacial Mountains in central Antarctica. From 12 samples we analysed detrital zircons for U–Pb ages, Hf-isotopes, and trace elements to determine the age, rock type and source of the host magma, and TDM model age. Clusters, in decreasing order of abundance, are (1) 820–1000 Ma, host magmas felsic granitoids with alkaline rock, (2) 1500–1700 Ma felsic granitoids, (3) 500 to 700 Ma mafic granitoids with alkaline rock, (4) 2400–2550 Ma granitoids, and (5) 1000–1200 Ma felsic and mafic granitoids, mafic rock, and alkaline rock. TDM ranges from 1.5 to 3.5 Ga. Joint paleoslope measurements and zircon ages indicate that the Eastern Ghats Mobile Belt (EGMB) and lateral belts and conjugate Antarctica are potential provenances. Zircons from the Gondwana Rifts differ from those in other Gondwanaland sandstones in their predominant 820–1000 Ma and 1500–1700 Ma ages (from the EGMB and conjugate Rayner–MacRobertson Belt) that dilute the 500–700 Ma (Pan-Gondwanaland) ages. The 1000–1200 Ma zircons reflect the assembly of Rodinia, the 500–700 Ma ones that of Gondwanaland; the other ages reflect collisions in the region.  相似文献   

16.
A structural RbSr isotopic study has been made on two small areas in the eastern Pilbara block, Western Australia. The sites were chosen because they showed good interrelationships of granitoid and greenstone units. At Warrery Gap, on the western side of the Corunna granitic dome, the acid volcanics of the Duffer Formation (at the top of the lower Talga-Talga Subgroup of the Warrawoona Group) show some updating, but a good 3506 ± 62 Ma isochron, with initial ratio (IR) of 0.7006 ± 0.0011, was recognized, consistent with the 3452 ± 16 Ma zircon measurement of Pidgeon (1978b).Both a penetrative D1 deformation, and the doming of D2 preceded intrusion of late tectonic granitoids, dated at 3270 ± 22 Ma (IR = 0.7015 ± 0.0003), into both the Duffer Formation and the overlying pillow lavas of the Salgash Subgroup. Thus, the Salgash Subgroup is much older than suggested by Glikson (1979) and the stratigraphic succession cannot contain a hiatus between the apparently conformable Talga-Talga and Salgash Subgroups of the magnitude he proposed.The granitic domes clearly owe their form to the D2 deformation rather than to batholitic intrusion, but near horizontal structural lineations suggest that they were not formed by diapiric movements. A granodiorite and pink feldspar granite from just within the Corunna granitic dome are slightly deformed: pooled isochrons indicate an age of 3232 ± 27 Ma but different IRs of 0.7032 and 0.7009, respectively. At Tambourah, in the eastern Shaw granitic dome, local D2 also deforms an intrusive microadamellite of age 3087 ± 34 Ma and IR = 0.7103 ± 0.0057. There is therefore a real spread in ages of D2 granitoids and D2 deformation between about 3300 and 3100 Ma.Layered megacrystic gneiss, at Tambourah, also intruded by the microadamellite, contains a nebulous foliation argued to be local S1. Layered and homogeneous megacrystic gneiss produce updated RbSr total rock isochrons of 2995 ± 95 and 2779 ± 38 Ma, respectively. The primary age of these D1 gneisses is clearly greater than that of the D2 granitoids and is probably indicated by Pidgeon's (1978c) zircon age of 3417 ± 40 Ma from the Shaw granitic dome. If so, Hickman's (1975) “Migmatite Suite” contains both D1 gneisses and D2 granitoids separated in age at Tambourah by ca. 300 Ma, although neither appears to be older than the lower part of the preserved layered sequence. The protocrust on which that sequence was deposited has yet to be directly identified.  相似文献   

17.
高利娥  曾令森  刘静  谢克家 《岩石学报》2009,25(9):2289-2302
藏南也拉香波穹隆位于近东西向展布的北喜马拉雅片麻岩穹隆(NHGD)最东端,主要由石榴角闪岩、石榴石云母片麻岩、二云母花岗岩和淡色花岗岩组成.SHRIMP锆石U/Pb定年结果表明也拉淡色花岗岩的结晶年龄为35.3±1.1Ma,明显老于位于该穹隆以西类似的淡色花岗岩(年龄普遍<25Ma).全岩元素和Sr-Nd同位素测试结果揭示：(1)也拉香波淡色花岗岩为过铝质富钠花岗岩;(2)与片麻岩相似,也拉香波淡色花岗岩富集大离子亲石元素(LILE,如K,Sr,Rb和Ba),但亏损Ti,Y,Yb,Sc和Cr;(3)和片麻岩或角闪岩相比,也拉香波淡色花岗岩同时亏损LREE和HREE,但与HREE相比,LREE相对富集;(4)在Sr-Nd同位素系统特征上.淡色花岗岩初始Sr同位素比值与角闪岩的相当,在0.711949～0.719344之间;但远小于片麻岩.而Nd同位素组成在片麻岩和角闪岩之间,在-8.9～-15.0之间.以石榴角闪岩和片麻岩为端元,简单混合计算表明：由石榴角闪岩为主和片麻岩为辅组成的混合源区发生部分熔融作用,各自产生的熔体进行不同程度的混合,可形成类似于也拉香波淡色花岗岩成分的岩浆,其中角闪岩的部分熔融起主要作用.使用Zr在岩浆中的饱和浓度温度计得出岩浆的平均温度为673℃,在此温度下,变泥质片麻岩在高压(～10kbar)条件下的水致部分熔融和角闪岩部分熔融都可形成也拉过铝质富钠淡色花岗岩,但角闪岩的脱水部分熔融起主导作用.在地壳增厚条件下,下地壳角闪岩的部分熔融可能是导致喜玛拉雅造山带从缩短增厚向伸展垮塌转换的主要因素之一.  相似文献   

18.
Field, structural, and metamorphic petrology investigations of Mabja gneiss dome, southern Tibet, suggest that contractional, extensional, and diapiric processes contributed to the structural evolution and formation of the domal geometry. The dome is cored by migmatites overlain by sillimanite-zone metasedimentary rocks and orthogneiss; metamorphic grade diminishes upsection and is defined by a series of concentric isograds. Evidence for three major deformational events, two older penetrative contractional and extensional events and a younger doming event, is preserved. Metamorphism, migmitization, and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid-crustal levels. Metamorphic temperatures and pressures range from 500 °C and 150–450 MPa in chloritoid-zone rocks to 705±65 °C and 820±100 MPa in sillimanite-zone rocks. We suggest that adiabatic decompression during extensional collapse contributed to development of migmatites. Diapiric rise of low density migmatites was the driving force, at least in part, for the development of the domal geometry. The structural and metamorphic histories documented in Mabja Dome are similar to Kangmar Dome, suggesting widespread occurrence of these events throughout southern Tibet.  相似文献   

19.
The largest gold district in China is the Jiaodong Peninsula, where three types of gold deposits are recognized: quartz vein, fracture-altered and breccia types. The first two developed along a group of NE-trending faults and are hosted by granitic intrusions, dated at 160 to 150 Ma (biotite granite) and 130 to 126 Ma (granodiorite), and by metamorphic rocks of the Precambrian crystalline basement. The breccia-type gold system is mainly located around the northern margin of the Jiaolai Cretaceous basin, where mineralisation is controlled by both detachment fractures and NE-trending faults. This study is based on stable isotope determinations from ten gold deposits, including Linglong, Jiaojia, Sanshandao, Cangshang, Wang'ershan, Dayigezhuang, Denggezhuang, Pengjiakuang, Fayunkuang and Dazhuangzi, as well as the Linglong Jurassic biotite granite, Guojialing Cretaceous granodiorite and Archean gneiss. The stable isotope systematics reflect the style of the three types of gold deposits, but also show that they belong to the same metallogenic system, in which the hydrothermal fluids were derived from a mantle fluid reservoir and mixed with crustal fluids. The ore-forming age is later than both the Jurassic biotite granite and Cretaceous granodiotite, but overlaps with the 121 to 114 Ma ages of lamprophyre and diabase dykes. The hydrothermal fluids that were responsible for both gold mineralisation and the retrograde alteration of the diabase and lamprophyre dykes are similar, and represent a CO2 and potassium-rich system. This fluid system is interpreted to be the consequence of Cretaceous lithospheric thinning, asthenospheric upwelling and mantle degassing in Eastern China.  相似文献   

20.
Precise U–Pb geochronology and Hf isotope tracing of zircon is combined with whole-rock geochemical and Sr and Nd isotope data in order to unravel processes affecting mafic to felsic calcalkaline magmas prior to and during their crystallization in crustal magma chambers along the southern border of Central Srednogorie tectonic zone in Bulgaria (SE Europe). ID-TIMS U–Pb dating of single zircons from felsic and mixed/mingled dioritic to gabbroic horizons of single plutons define crystallization ages of around 86.5–86.0, 85.0–84.5 and 82 Ma. Concordia age uncertainties are generally less than 0.3 Ma (0.35%–2σ), and as good as 0.08 Ma (0.1%), when the weighted mean 206Pb/238U value is used. Such precision allows the distinction of magma replenishment processes if separated by more than 0.6–1.0 Ma and when they are marked by newly saturated zircons. We interpret zircon dates from a single sample that do not overlap to reflect new zircon growth during magma recharge in a long-lived crustal chamber. Mingling/mixing of the basaltic magma with colder granitoid mush at mid- to upper-crustal levels is proposed to explain zircon saturation and fast crystallization of U- and REE-rich zircons in the hybrid gabbro.Major and trace-element distribution and Sr and Nd whole-rock isotope chemistry define island arc affinities for the studied plutons. Slab derived fluids and a sediment component are constrained as enrichment sources for the mantle wedge-derived magma, though Hf isotopes in zircon suggest crustal assimilation was also important. Inherited zircons, and their corresponding ε-Hf, from the hybrid gabbroic rocks trace the lower crust as possible source for enrichment of the mantle magma. These inherited zircons are about 440 Ma old with ε-Hf of − 7 at 82 Ma, whereas newly saturated concordant Upper Cretaceous zircons reveal mantle ε-Hf values of + 7.2 to + 10.1. The upper and middle crusts contribute in the generation of the granitoid rocks. Their zircon inheritance is Lower Palaeozoic or significantly older and crustal dominated with 82–85 Ma corrected ε-Hf values of − 28. The Cretaceous concordant zircons in the granitoids are mantle dominated with a ε-Hf values spreading from + 3.9 to + 7.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号