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1.
The Ghanzi-Chobe Belt defines a linear zone of volcano-sedimentary rocks that form inliers through the Kalahari Desert of western and northern Botswana. Geological investigations along the Ghanzi Ridge have revealed a sequence composed of a basal bimodal volcanic suite with minor intercalated sedimentary rocks, termed the Kgwebe Formation, and an upper siliciclastic sedimentary unit with subordinate carbonates, called the Ghanzi Group. To the northeast in the Chobe District, basal volcanics informally termed the Goha Hills Formation are overlain by carbonate-bearing siliciclastics assigned to the Chinamba Hills Formation. Geochemical analysis and field relations of the Kgwebe Formation volcanics indicate that the sequences were accumulated in a continental rift basin. Rifting was initiated by extensional tectonics associated with a continental collision along the Namaqua-Natal Belt. The Kgwebe Formation volcanics started to accumulate during Mesoproterozoic times about 1 106 Ma ago. The depositional basin developed from fluvial and lacustrine systems during the accumulation of the Kgwebe Formation, into an extended shallow marine basin that accommodated the Kuke, Ngwako Pan, D'kar, and Mamuno Formations of the Ghanzi Group. Stratabound copper-sulfide mineralisation was developed at a redox interface within the Ghanzi Group. Mineralisation is confined to the basal part of the grey-green, argillitic facies of the D'kar Formation and is either disseminated or is found along cleavage planes and fractures. Minor Cu mineralisation also occurs in carbonates of the Chinamba Hills Formation. Tectonic deformation during the Pan African Damaran Orogeny resulted in a fold and thrust belt known as the Ghanzi-Chobe Belt.  相似文献   

2.
Ion microprobe UPb ages of 1104 ± 16 Ma, 1363 ± 11 Ma and 1748 ± 13 Ma have been obtained on detrital zircons from the siliciclastic rocks of the Ghanzi Group intersected in borehole CKP4 in northern Botswana. Available data show that the time of deposition of the Ghanzi Group is bracketed between 1106 ± 2 and 627 ± 6 Ma. These ages permit constraints to be placed on the source of the sediments. It is shown that the main component of the sediments is sourced locally, from the Kibaran-age Kgwebe igneous rocks. These data are taken to indicate that the Kibaran orogenic event affected northwest Botswana. The Kibaran-age lithological units of northern Botswana represent a link between the Kibaran-age belts of central-eastern Africa and southern Africa. The other two populations of zircons are probably exotic. One is linked to the ca. 1400-1300 Ma rocks known within the Kibaran orogenic system in central and southwest Africa, possibly the Choma-Kalomo Block in Zambia, whereas the ca. 1750 Ma detrital zircons were probably sourced from Palæproterozoic granitoids exposed in northern Namibia and southern Angola.  相似文献   

3.
A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District.New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana.Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends.Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness.  相似文献   

4.
华北克拉通南缘豫西地区保存有较为完整的变质结晶基底和中-新元古代沉积盖层,记录了重要的前寒武纪构造演化信息。近年来的年龄研究结果表明原认为是中-新元古代的汝阳群-洛峪群可能形成于中元古代早期(1. 75～1. 60Ga),而沿着华北克拉通南缘与秦岭造山带的拼合带(洛南-栾川断裂带)分布的新元古代盖层(主要为栾川群)的形成时代尚不明确。华北南缘新元古代栾川群主要由大理岩、片岩、千枚岩和碱性火山岩组成,其上部大红口组火山岩以碱性粗面质岩石为主,高硅富钾,与侵入到栾川群中下部的辉长岩构成典型的双峰式岩石组合。栾川群大红口组三个粗面岩样品的锆石U-Pb年龄分别为840±4Ma、845±5Ma和846±6Ma,结合已有的栾川群下伏地层最年轻的碎屑锆石年龄结果(～1000Ma),限定栾川群归属于新元古代早期(1000～840Ma)。大红口组岩浆岩的岩石组合和地球化学特征表明其形成于板内裂谷环境,根据区域构造资料以及前人的研究成果,栾川群火山岩与北秦岭同时期的岩浆活动共同指示了华北南缘与北秦岭在～845Ma均处于板内拉张阶段。  相似文献   

5.
We report here new field and analytical data from Precambrian rocks on Hainan Island of the Cathaysia Block, south China, and examine its probable connection to Laurentia. Granitoids and newly discovered felsic volcanic rocks dated at 1433 ± 6 Ma and 1439 ± 9 Ma (SHRIMP U‐Pb zircon) on Hainan Island are coeval with, and isotopically similar to the 1500–1350 Ma trans‐continental granite‐rhyolite province in southern Laurentia. Quartzites unconformably overlying the ca. 1430 Ma volcaniclastic rocks on Hainan Island are interpreted as locally‐sourced Grenvillian foreland basin deposits that can be correlated with the Deer Trail Group of south‐western Laurentia. The detrital provenance of the quartzites contains age populations comparable to the 1610–1490 Ma, westerly‐sourced non‐Laurentian detrital grains reported in the Belt Basin of south‐western Laurentia. Our new data thus make Cathaysia the most likely continental block next to western Laurentia before and during the late Mesoproterozoic assembly of Rodinia.  相似文献   

6.
A comprehensive review of new data on geology and geochronology of Precambrian terranes in the western Central Asian Orogenic Belt reveals new insights into its evolution. At the present surface, these terranes mostly consist of Meso- to Neoproterozoic sedimentary, magmatic and metamorphic assemblages, with insignificant Paleoproterozoic rocks. Archean material is represented exclusively by detrital and xenocrystic zircons in younger strata. Meso- to Neoproterozoic felsic magmatic rocks were mostly sourced from Neoarchean and Paleoproterozoic continental crust, indicating its reworking and potential wider presence at deeper crustal levels. Most Meso- to Neoproterozoic assemblages are of intraplate origin. The supra-subduction assemblages of Neoproterozoic and Mesoproterozoic ages are of limited extent.We propose to recognize the Issedonian and Ulutau-Moyunkum groups of terranes, separated by early Paleozoic Z-shaped ophiolitic suture, based on their different tectono-magmatic evolution in the Mesoproterozoic and Neoproterozoic. Distinctly different are the Mesoproterozoic and early Neoproterozoic assemblages, with lithological variations at the beginning of the late Neoproterozoic and practically no differences at the end of the Neoproterozoic.The Issedonian group of terranes could be part of a Mesoproterozoic (ca. 1100 Ma) orogen between the Siberian, North China and Laurentian cratons. The pre-Mesoproterozoic crust of these terranes was completely reworked during the younger events. The Ulutau-Moyunkum group of terranes appear to be lithologically and geochronologically similar to the Tarim craton. Both the Issedonian and Ulutau-Moyunkum groups of terranes were metamorphosed during the Ulutau-Moyunkum event at 700 ± 25 Ma.The breakup into currently mappable Precambrian terranes took place during end-Ediacaran to early Paleozoic times after opening of oceanic basins, whose relics are preserved in numerous Paleozoic ophiolitic sutures.  相似文献   

7.
In a re-study of regional geology by the China Geological Survey (CGS), the key problem is in the stratigraphical division and correlation. According to the new isotopic dating of the Meso- and Neoproterozoic in China, there have been great changes in the strata correlation and tectonic explanation. The authors obtained four zircon sensitive high resolution ion micro-probe (SHRIMP) U-Pb datings from the bentonite of the Lengjiaxi Group (822±10 Ma, 823±12 Ma and 834±11 Ma) and Banxi Group (802.6±7.6 Ma) in north Hunan Province, which is considered to be the middle part of the Jiangnan Orogenic Belt. On the basis of the zircon dating mentioned above, the end of the Wuling orogen is first limited in the period from 822 Ma to 802 Ma in one continued outcrop (Lucheng section) in Linxiang city, Hunan Province. Combining a series of new zircon U-Pb datings in the Yangtze and Cathaysia blocks, several Neoproterozoic volcanic events and distribution of the metamorphic rocks in the Jiangnan Orogenic Belt have been distinguished. In the context of the global geodynamics, it is useful to set up a practical and high precision chronological framework and basic and unified late Precambrian section in South China.  相似文献   

8.
赣东北婺源-德兴地区新元古代地层中浅变质火山岩主要由变质玄武岩、英安岩和流纹岩组成.全岩地球化学分析表明浅变质玄武岩具有拉斑玄武岩的地球化学特征,起源于尖晶石辉橄岩低度部分熔融,英安岩岩浆起源于壳源杂砂岩部分熔融,流纹岩可能为英安质岩浆结晶分异的产物.LA-ICP-MS锆石U-Pb同位素定年揭示婺源浅变质英安岩形成于861±8Ma,德兴张村西浅变质流纹岩形成于860±3Ma,铜厂铜矿矿区凝灰质板岩形成于860±6Ma,均为早新元古代Tonian期.同时这些定年样品中保存了2.8 ～2.5Ga、2.0～1.7Ga和～1.0Ga的继承或捕获锆石记录.结合浅变质玄武岩和英安质火山岩的地球化学特征和成因,这套岩石最有可能形成于新元古代早期安第斯型活动大陆边缘弧后盆地构造背景.  相似文献   

9.
彭润民  王建平 《地学前缘》2020,27(2):420-441
狼山-渣尔泰山是华北克拉通北缘西段元古宙伸展构造体制下被动陆缘的重要热水喷流成矿带,内产东升庙、炭窑口、霍各乞、甲生盘等大型-超大型矿床,其含矿建造是元古宇渣尔泰山群。该成矿带的主体在狼山山脉主峰地区,产有东升庙、炭窑口、霍各乞三大矿床。自2010年在狼山西南原渣尔泰山群阿古鲁沟组地层中发现新元古代酸性火山岩(锆石年龄816~805 Ma)以来,其东部狼山主峰地区的渣尔泰山群及产在其中的炭窑口、东升庙、霍各乞矿床是否也都可以归于新元古代形成是需要进一步研究的重要科学问题。本文依据的事实有:(1)狼山西南新元古界的碎屑锆石最小年龄为1 155 Ma与碳酸盐岩是方解石大理岩;(2)炭窑口矿区渣尔泰山群顶部刘鸿湾组地层的碎屑锆石年龄的两个年龄峰值分别为1 862~1 762 Ma(最小年龄为1 732 Ma)和2 448 Ma,所含碳酸盐岩全部是白云石大理岩类;(3)狼山北侧狼山群含矿岩组中发现年龄为887 Ma±的新元古代基性火山岩; (4)霍各乞矿床含重晶石与微晶长石的硅质层发育,多与黄铁矿层互层状产出,也有与闪锌矿+磁黄铁矿及方铅矿层互层,激光原位分析得到的黄铁矿、方铅矿、闪锌矿和磁黄铁矿的δ34S为17.60‰~21.97‰,显示喷流成矿的硫同位素组成特征。根据以上事实可以确认:(1)狼山西南含酸性火山岩与方解石大理岩的前寒武纪地层是与狼山南侧炭窑口矿区含白云石大理岩的渣尔泰山群在不同盆地中沉积而成;(2)狼山南侧是中元古代裂谷带,产有以炭窑口、东升庙矿床为代表的中元古代喷流-沉积成矿系统;(3)狼山北侧是新元古代裂谷带,产有以霍各乞矿田为代表的新元古代热水喷流成矿系统。产在新元古代狼山群中的霍各乞矿是介于SEDEX型与VMS型之间、但靠近SEDEX型一侧的热水喷流-沉积矿床。  相似文献   

10.
杨红  刘福来  刘平华  王舫 《岩石学报》2013,29(6):2161-2170
大红山群是扬子地块西南缘出露的古元古代结晶基底,主要经历了绿片岩相-低角闪岩相变质作用.本研究对大红山群老厂河组变质中酸性岩和变质沉积岩——石榴白云母-长石石英片岩中的白云母进行了40Ar-39Ar测年,得到三个样品的坪年龄和40Ar/39Ar等时线年龄结果较统一,坪年龄代表的变质年龄分别为837.7±4.2Ma、839.6±4.2Ma和844.2±4.2Ma.变质沉积岩和变质中酸性岩的变质时代类似,均介于837～845Ma.大红山群变质基性岩中变质锆石的U-Pb定年年龄为849±12Ma(杨红等,2012),40Ar-39Ar测年数据与锆石定年数据相结合,说明大红山群古元古代结晶基底中的火山岩和沉积岩均在新元古代经历了同期变质作用,其主期低角闪岩相变质作用发生于新元古代837～850Ma.结合前人发表的扬子西缘～750Ma的变质年龄,扬子西缘从北向南的区域变质作用时限可扩展到750 ～850Ma.此外,扬子西缘存在750～850Ma的岩浆事件,本文研究结果说明,扬子地块西缘在新元古代不仅发生了大规模岩浆作用,也发生了750～850Ma的区域变质作用,扬子西缘存在新元古代的岩浆-变质事件.岩浆事件与变质事件之间可能存在相关性,即新元古代岩浆作用引起了扬子西缘的区域动力热流变质作用.  相似文献   

11.
朝鲜半岛平南盆地中元古代岩浆事件   总被引：4，自引：2，他引：2  
朝鲜平南盆地翁津地区发育中元古代黄海群和同时期(称之为瓮津期)花岗岩,花岗岩体侵入于黄海群。本文采用锆石原位微区U-Pb定年技术,对黄海群中的酸性火山岩及花岗岩进行了年龄测试。获得的数据表明,黄海群中下部层位及上部层位的酸性火山岩分别在1235±5Ma和1203±7Ma喷发,由此说明黄海群的沉积时代应为中元古代,而不是传统上认为的古元古代;两个翁津期花岗岩体(翁津和黄衣山岩体)的侵位年龄分别为1251±22Ma和1248±13Ma,为中元古代花岗质岩浆活动的产物。上述1251~1203Ma年龄的获得,表明朝鲜半岛发育中元古代岩浆作用,从而明确朝鲜黄海裂谷与华北东缘裂谷在时间上具有同期性,同时也表明中国华北与朝鲜在中元古代具有类似的发展历史。  相似文献   

12.
桃溪群混合岩位于南岭东段武夷山褶皱带南段.本文通过采自桃溪附近帽村-田园两个代表性花岗片麻岩样品进行LA-ICP-MS锆石U-Pb 测年获得494～496Ma的结晶年龄,641～657Ma和799Ma两组继承锆石的206Pb/238U 表观年龄.同时对样品FJ-137进行的Hf同位素测定表明其εHf(t)变化于-2.3...  相似文献   

13.
The late Mesoproterozoic to Neoproterozoic Kalahari Copperbelt (KCB) in Namibia and Botswana is widely covered by Kalahari sand, which precludes direct correlations between known stratabound sediment-hosted Cu–Ag districts. We use a combination of review of literature data, and newly processed and interpreted high-resolution aeromagnetic maps in both countries to provide a new correlative cross-border interpretation. Lithostratigraphic control on the aeromagnetic response allows detailed indirect mapping of the Kalahari Copperbelt lithotectonic domains below the sand cover. This enabled us to redefine the width and lateral extent of the KCB as two continuous magnetic domains (the Rehoboth and Ghanzi–Chobe domains) extending from central Namibia to northern Botswana, and helped in resolving problems of stratigraphic correlations across the international border.The Rehoboth magnetic domain, in the western part of the KCB in Namibia, records continental arc magmatism at ~ 1200 Ma during orogenic events along the northwestern edge of the Kalahari Craton. This was followed at 1110–1090 Ma by widespread magmatism, identified within the entire KCB, and related to the 1112–1106 Ma Umkondo Large Igneous Province. The basal parts of the Tsumis Group in Namibia and Ghanzi Group in Botswana were deposited in shallow-water environments after a period of erosion and peneplanation. Subsequently, and prior to the Sturtian glaciation, the host-rocks of the Cu–Ag deposits formed by the deposition of chemically reduced shales and siltstones that formed in deeper water and overlie chemically oxidised shallow-water sandstones. This regional interface, which is both a permeability barrier and redox boundary, played a critical role in the formation of the stratabound sediment-hosted Cu–Ag deposits of the Kalahari Copperbelt, and the interface, with its strong magnetic contrast, can be followed through the entire Ghanzi–Chobe magnetic domain of the copperbelt. The whole KCB was affected by the Damara Orogeny during early Cambrian times, which resulted in the formation of a NE–SW trending ~ 250 km-wide fold-and-thrust belt.  相似文献   

14.
塔里木盆地北部志留系碎屑锆石测年及其地质意义   总被引：3，自引：0，他引：3  
志留系是塔里木盆地第一套砂岩储层广泛分布的沉积盖层,其沉积来源与成因对志留纪构造演化及周边造山带的研究具有重要意义。塔里木盆地北部地区2个志留系代表性样品的碎屑锆石LA-ICP-MS U-Pb定年研究表明,志留系具有比较集中的三期物源年龄:中元古代早期1500～1600Ma、新元古代早期750～900Ma、奥陶纪450～500Ma。碎屑锆石定龄表明东部地区志留系物源主要来自阿尔金地区奥陶纪火成岩,而西部塔北地区志留系物源主要来自北部古隆起前寒武纪基底。前寒武纪锆石年龄揭示塔里木板块在新元古代时期与Rodinia超大陆具有相似的聚合与裂解演化史,塔里木北部地区在中元古代存在与Columbia超大陆裂解时间一致的构造-热事件。  相似文献   

15.
The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts.Here,we report new petrological,geochemical,and zircon U-Pb geochronological data of various metamorphic rocks(felsic to mafic orthogneiss,pelitic schist,and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe,and evaluate the timing and P-T conditions of the collisional event as well as protolith formation.Geochemical data of felsic orthogneiss indicate within-plate granite signature,whereas those of mafic orthogneiss suggest MORB,ocean-island,or within-plate affinities.Metamorphic P-Testimates for orthogneisses indicate significant P-T variation within the study area(700-780 C/6.7-7.2 kbar to 800-875 C/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks.Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813士5 Ma,which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism.Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic(ca.2700-750 Ma).The Neoarchean(ca.2700-2630 Ma) and Paleoproterozoic(ca.2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe,respectively.The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic(ca.1500-950 Ma) and early Neoproterozoic(ca.900-750 Ma) detrital zircons,respectively.Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic(744-670 Ma),and subsequently underwent highgrade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian.In contrast,670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports,but consistent with~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen,suggesting Cryogenian thermal events before the final collision.  相似文献   

16.
The Saldania Belt (SB), located in the southernmost part of South Africa, contains S-, I-, and A-type granites. Whole-rock Sm?CNd data for the Saldania granites indicate the presence of a juvenile as well as inherited crustal signature. The earlier S-type granites have ??Nd(t) values from ?4.2 to ?3.28 (for t?=?550?Ma). In contrast, the intermediate I-type and youngest A-type and highly fractionated I-type granites display ??Nd values ranging from ?1.44 to ?3.68 (for t?=?540?Ma) and from +3.66 to +5.1(for t?=?530?Ma), respectively. The U?CPb single zircon data of A-type granites exposed in the Western Branch of the SB yielded dates from 524?±?8 to 510?±?4?Ma, whereas an S-type granite, situated in the Southern Branch of the SB and represented by the syn- to late-tectonic Rooiklip Granite, yielded an age of 527?±?8?Ma. The volcano-sedimentary rocks intruded by these granites display Nd model ages from Ga to 1.67?Ga and ??Nd(t) values from ?6.58 to +3.34 (for t?=?560?Ma) with isotope signature similar to those of the granites. The S- and I-type granitic magmatism is mostly a product of melting of an earlier crust (Mesoproterozoic to Paleoproterozoic) with different degree of juvenile contribution. The obtained isotope data and field relationship support the hypothesis that the lithological units of the SB were affected by the late Neoproterozoic to Early Cambrian tectonism, related to compressive deformational processes at the southern margin of the Kalahari Plate and probably correlated with the Sierra La Ventana Belt basement.  相似文献   

17.
秦岭岩群被认为是出露于北秦岭地体内最古老的前寒武纪基底岩石,记录了北秦岭造山带的地壳形成和演化历史。本文报道丹凤-西峡地区五件秦岭岩群片麻岩锆石U-Pb年龄结果,限定其形成和变质时代,探讨北秦岭地体的构造归属。定年结果表明,岩浆成因锆石颗粒的年龄集中在1400～1600Ma左右和850～950Ma左右,记录两期主要岩浆活动。6粒锆石具有变质成因特征,低Th/U比值(0.03),206Pb/238U年龄变化在510～465Ma之间,加权平均值477±18Ma。这一古生代变质叠加时代与北秦岭地体南北缘高压变质作用时代基本一致,说明秦岭岩群遭受到北秦岭造山带俯冲-碰撞造山过程的变质作用。秦岭岩群主要形成于中元古代晚期至新元古代早期,基底岩石缺乏早元古代和太古代岩浆活动的记录。在岩浆作用时代上,北秦岭地体与广泛发育新元古代中-晚期岩浆作用的扬子陆块北缘有差别,也不同于晚太古代-早元古代的华北陆块南缘,可能是中-新元古代形成的独立微陆块。  相似文献   

18.
黔东南1∶5万高武幅、宰便幅区域地质调查结果表明,新元古界下江群是一套沉积超覆于中元古界四堡群之上的裂谷系楔状地层,其底部甲路组沉积底砾岩高角度(不整合)沉积超覆于四堡群复理石浊积岩之上,或沉积超覆于侵入四堡群之中的摩天岭花岗岩之上。取自该地区沉积超覆面之下摩天岭花岗岩样品的TIMS锆石U-Pb同位素年龄为825.0±2.4Ma,表明该地区新元古代裂谷系开始接受沉积的时间应该晚于825±2.4Ma;而取自沉积超覆面之上甲路组底部同沉积基性火山岩样品的TIMS锆石U-Pb同位素年龄为814±13Ma,这一年龄大致代表了该地区新元古代沉积超覆的开启时间,且与目前已获得的华南其它地区新元古代裂谷系沉积超覆的开启时间(820Ma)十分接近。本项研究成果支持华南裂谷系沉积超覆的开启时间为820Ma左右的观点。  相似文献   

19.
The geodynamic nature of the Late Neoproterozoic island-arc dacites (691 ± 8.8 Ma) and rift basalts (572 ± 6.5 Ma) of the Kiselikhinskaya Formation, Kutukasskaya Group, in the Isakovskii terrane is inferred from geochemical data and U–Pb zircon (SHRIMP-II) dates. The volcanic rocks were produced during the late evolutionary history of the Yenisei Range, starting at the origin of oceanic crustal fragments and their accretion to the Siberian craton to the postaccretionary crustal extension and the onset of the Caledonian orogenesis. The reproduced sequence of geological processes marks the early evolution of the Paleo- Asian Ocean in its junction zone with the Siberian craton. The data refine the composition and age of volcanic rocks in the trans-Angara part of the Yenisei Range and specifics of the Neoproterozoic evolution of the Sayan–Yenisei accretionary belt.  相似文献   

20.
杨敏  刘良  王亚伟  廖小莹  康磊  盖永升 《岩石学报》2016,32(5):1452-1466
碎屑锆石为沉积岩中最稳定的矿物,其年龄谱系不仅可限定沉积物的最大沉积年龄与物源性质,而且能够为探讨其形成构造环境提供关键约束。作为北秦岭构造带主要构造岩石地层单元之一,二郎坪杂岩由北、中、南三个构造岩片——北部碎屑岩片、中部蛇绿岩片和南部变碎屑岩片组成。本文对二郎坪杂岩南部变碎屑岩片中的碎屑锆石进行了LA-ICP-MS U-Pb年龄测试研究,获得样品锆石谐和年龄值分布在500±7Ma~3894±5Ma之间,1个年龄高频集中区为0.9~1.0Ga,6个次要年龄集中区分别为500~600Ma、750~850Ma、1.35~1.48Ga、1.6~1.75Ga、2.6~2.7Ga和3.0~3.4Ga。研究结果表明:1)最小峰值年龄限定二郎坪杂岩南部碎屑岩片原岩的最大沉积时代为500Ma,结合西庄河花岗闪长岩侵入南部岩片的地质事实,南部岩片的形成时代应为早古生代寒武-奥陶纪(500~475Ma),明显早于中部蛇绿岩片中的火山岩的形成时代(463~475Ma);2)通过与邻区地质事件年龄谱峰及其锆石阴极发光图像特征的对比,二郎坪杂岩变碎屑岩原岩的物源分别主要来自南部秦岭杂岩中的早新元古代花岗岩和北部华北南缘熊耳群中元古代火山岩,其物源具有双源性,暗示其形成于与伸展作用相关的裂谷或弧后盆地构造背景,而二郎坪蛇绿岩片所代表的古洋盆可能正是在此基础上发育产生的;3)结合区域地质背景资料分析,二郎坪杂岩中的沉积碎屑岩片可能形成于商丹洋向北俯冲期间所产生的弧后伸展盆地构造环境;4)通过与宽坪岩群沉积岩中的碎屑锆石年龄数据的对比,揭示二者碎屑锆石具有相似的年龄谱峰,结合宽坪群变沉积岩中发现大量疑源类、几丁虫和虫颚等早-中奥陶世化石组合以及二郎坪杂岩蛇绿岩片中的火山岩夹层硅质岩中发现早-中奥陶世牙形石和放射虫的研究,分析认为二者沉积物的沉积时代相近,沉积物源几乎完全一致,暗示它们形成的构造环境可能具有一致性;5)二郎坪蛇绿岩片中的典型的与洋壳俯冲有成因联系的弧火山岩的形成时代(463~475Ma)明显迟后于区内高压-超高压岩石的峰期变质时代(514~484Ma)约20~30Myr,清楚地表明二郎坪洋壳拖曳秦岭杂岩发生陆壳俯冲-深俯冲作用的可能性不大;6)二郎坪杂岩南部碎屑岩片和宽坪岩群碎屑沉积物中碎屑锆石中最主要的年龄集中区(0.9~1.0Ga)的物源来自秦岭杂岩中的早新元古代花岗质岩石,而缺少秦岭杂岩中的早古生代岩浆岩和HP/UHP变质岩石锆石的年龄信息,明确指示二郎坪与宽坪盆地沉积时,秦岭杂岩重要组成的早新元古代花岗质岩石已出露于地表接受剥蚀,而秦岭杂岩中出露的HP/UHP岩石和早古生代岩浆岩未抬升出露地表,即秦岭杂岩现今出露的前早古生代陆壳物质不是整体而是部分经历了陆壳俯冲-深俯冲作用。  相似文献   

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