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1.
The petrography and major and trace element concentrations of the sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, are studied to determine their provenance, intensity of weathering and tectonic setting. The detrital compositions of the Tumengela sandstone samples are dominated by quartz (58.0–70.1 %, average 64.7 %) and lithic fragments (21.8–35.9 %, average 27.3 %), but low in feldspar content (4.9–12.9 %, average 8.0 %). The sandstones can be classified as litharenite and feldspathic litharenite according to their detrital compositions, which is consistent with the geochemical data. The detrital modal compositions reflect that these sandstones are probably derived from a recycled orogenic source. The index of chemical variability (ICV) and SiO2/Al2O3 ratio values suggest that the compositional maturity and recycling were moderate. The weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and Al2O3–(CaO* + Na2O)–K2O (A–CN–K) diagram indicate that the intensities of weathering in the source area were moderate. The Al2O3/TiO2, Th/Co, La/Sc, La/Co, Th/Sc, Cr/Th ratio values and the discriminant function of the Tumengela sandstones indicate that the sediments were mainly derived from felsic source rocks, while also mixed with intermediate source rocks. The comparison of rare earth element patterns and its Eu anomalies to the probable source rocks infer that the sandstones were derived from the combination of granite, rhyolite, dacite, and gneisses. The proximal central uplift belt was probably the primary provenance area as evidenced by the petrographical and geochemical features of the Tumengela sandstones. The multidimensional tectonic discrimination diagram based on major elements show a collision setting (80 %) combined with a rift setting (20 %) for the Tumengela sandstones, which is consistent with the general geology of the study areas.  相似文献   

2.
The Bétaré-Oya Gold District is one of the three gold districts in East Cameroon with varied types of mining. The study site is an open-cast mine located in an alluvial plain. The strata succession globally consists of two positive sequences represented by three facies associations and seven lithofacies: massive clast-supported(Gcm), massive trough-cross bedded(Gtm), and massive matrix-supported(Gmm)conglomerates; massive sands(Sm); and massive(Fm), heterolothic(Fhl), and convolute and heterolit...  相似文献   

3.
Petrography and geochemistry(major, trace and rare earth elements) of clastic rocks from the Late Palaeozoic Madzaringwe Formation, in the Tshipise-Pafuri Basin, Northern South Africa, have been investigated to understand their provenance. Sandstone petrography and detrital modes indicates that the Late Palaeozoic succession was derived from craton interior and recycled orogen provenance. Sandstones in the Madzaringwe Formation are sub-arkosic to sub-litharenite. The sediments may represent a recycled to craton interior provenance. The geochemical data of major elements show that sandstone and shales have the same source. The study of paleoweathering conditions based on modal composition, chemical index of alteration(CIA) and A-CN-K(Al2O3-Ca O+Na2O-K2O) relationships indicate that probably chemical weathering in the source area and recycling processes have been more important in shale and sandstone rocks. The relatively high CIA values(70–90%) indicates moderate to high weathering conditions of the samples and the paleoclimate of the source area was warm. K2O/Na2 O versus Si O2 and Na2O-Ca O-K2 O tectonic setting discrimination plots, suggest a passive continental margin. In the study of trace elements, triangular Th-Sc-Zr/10 and La-Th-Sc plots both suggest a passive margin setting of the basin. Petrographic and geochemical results of the samples suggest uplifted basement source areas dominated by sedimentary rocks and/or granite-gneiss rocks. The source rocks might have been the recycled pre-Soutpansberg Karoo Supergroup rocks and the metasedimentary rocks of the Soutpansberg Group. Other source rocks may have been the pre-Beit-Bridge basement rocks(granites and gneisses).  相似文献   

4.
Basement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra‐subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc‐basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks.  相似文献   

5.
The Devonian–Carboniferous Tsetserleg terrane of Mongolia forms part of the complex Central Asian Orogenic Belt (CAOB). The Tsetserleg terrane consists mainly of clastic sediments, and is situated in the southern Hangay–Hentey Basin. Internally the terrane is divided into the Erdenetsogt (Middle Devonian), Tsetserleg (Middle‐Upper Devonian) and Jargalant (Lower Carboniferous) Formations. Provenance and tectonic setting of the Hangay–Hentey Basin remains controversial, with proposals ranging from passive margin through to island‐arc. A suite of 94 Tsetserleg sandstones and mudrocks was collected with the aim of constraining provenance, source weathering, and depositional setting, using established petrographic and whole‐rock geochemical parameters. Petrographically the sandstones are immature, with average compositions of Q22F14L64, Q14F17L69, and Q18F12L70 in the Erdenetsogt, Tsetserleg, and Jargalant Formations, respectively. Lv/L ratios range from 0.81 to 1.00 (average 0.95), and P/F from 0.68 to 0.93 (average 0.83). Framework compositions indicate deposition in an undissected or transitional arc. Geochemically, the sandstones are classified as greywackes. Geochemical contrasts between sandstone and mudrock averages in each formation are small, with lithotype means for SiO2 ranging only from 65.54 to 68.62 wt.%. These features and weak trends on variation diagrams reflect the immaturity of the sediments. Comparison of elemental abundances with average upper continental crust, major element discriminant scores, and immobile element ratios indicate a uniform average source composition between dacite and rhyolite. The maximum value for the Chemical Index of Alteration in the Erdenetsogt Formation is about 78 after correction for K‐metasomatism, indicating moderate source weathering. Lower maximum values (61 and 63, respectively) in the Tsetserleg and Jargalant Formations indicate they were derived from a virtually unweathered and tectonically active source. Tectonic setting discrimination parameters indicate a continental island‐arc environment, similar to several other CAOB suites of similar age. This arc source may have been built on a continental fragment situated within the Mongol–Okhotsk Ocean during Middle Devonian‐Lower Carboniferous time.  相似文献   

6.
Yukio  Isozaki 《Island Arc》1997,6(1):2-24
Abstract Proto-Japan originated from a continental margin of the Neoproterozoic Yangtze (South China) craton. It represents a unique Permo-Triassic tectonic setting in western Panthalassa, where two distinct types of orogenic belt occurred side by side. There was an accretionary orogen between the Yangtze craton and the Proto-Pacific (Farallon) Plate and a collisional orogen between the Sino-Korean (North China) and Yangtze cratons. This article reviews results of the latest on-land geological studies concerning Permo-Triassic tectonics in Japan and proposes a new plate tectonic interpretation as well as a paleogeographic reconstruction of this particularly unique geotectonic regime. Special emphases are given to (i) the accretion processes and products derived by collision-subduction of the Permian Akiyoshi paleoseamount and Maizuru paleo-oceanic plateau; (ii) the field occurrence of 220-Ma Sangun high-P/T schists and its implication for the exhumation process and 'tectonic sandwich' structure; (iii) the extensive development of a subhorizontal nappe of the pre-Jurassic rocks and their bearing on the orogenic edifice; and (iv) the restricted occurrence of the 250-Ma collision complex in the Hida and Oki belts and the relevant connection to the Precambrian cratons and collision suture in East Asia. The newly proposed paleogeographic reconstruction is also tested by faunal provinciality of Permo-Triassic fossils from shallow-water sediments.  相似文献   

7.
We report Permian (ca. 272 Ma ±5.4 Ma) felsic dykes that intrude into the Neoproterozoic (ca. 750 Ma) magmatic suite of the Nagar Parkar Igneous Complex (NPIC), the western extension of the Malani Igneous Suite (MIS). The NPIC consists of Neoproterozoic basement amphibolites and granites (riebeckite–aegirine gray granites and the biotite–hornblende pink granites), all of which are intruded by several generations of mafic and felsic dykes. Granitic magmatism occurred in the Late Neoproterozoic (ca. 750 Ma) due to the subduction‐, followed by the rift‐related tectonic regime during the breakup of the Rodinia supercontinent. U–Th–Pb zircon and monazite CHIME age data of 700–800 Ma from the earlier generation porphyritic felsic dykes suggest the dyke intrusion was coeval or soon after the emplacement of the host granites. Our findings of Permian age orthophyric felsic dykes provide new insights for the prevalence of active tectonics in the MIS during late Paleozoic. Textural features and geochemistry also make the orthophyric dykes distinct from the early‐formed porphyritic dykes and the host granites. Our newly obtained age data combined with geochemistry, suggest the existence of magmatism along the western margin of India (peri‐Gondwana margin) during Permian. Like elsewhere in the region, the Permian magmatism in the NPIC could be associated with the rifting of the Cimmerian micro‐continents from the Gondwana.  相似文献   

8.
The sandstones of the Ridge and Athleta members of Chari Formation(Callovian-Oxfordian)exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-angular to sub-rounded, and show low to medium sphericity.These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic environment with higher PCO_2. Generally good to strong correlations between Al_2O3 and other oxides in these sediments indicate clay mineral control. The K_2O/Na_2O versus SiO_2 diagram indicates that the studied samples occupy passive margin fields but the SiO_2/Al_2O_3 versus K_2O/Na_2O plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field.  相似文献   

9.
Study indicates that the major paleocurrent and source direction for the Chang 8 Member of the Yangchang Formation, Upper Triassic in the Xifeng area of the southwestern Ordos Basin derived from the southwest direction with the southeast source as the subordinate one. While the Chang 6 Member was influenced not only by the same source as that of the Chang 8 Member from the southwest and the southeast direction, but also affected by the northeast and the east provenance around the Ordos Basin, based upon measurement of paleocurrents on outcrops located in the periphery Ordos Basin, analysis of framework grains and heavy minerals in sandstones of the Chang 6 and Chang 8 Members and their spatial distribution in the study area, combined with characteristics of trace elements and rare-earth elements of mudstones and of a small amount of sandstones in the Xifeng area and outcrops in margin of the Ordos Basin. The Yuole-Xuanma-Gucheng-Heshui-Ningxia region located in the northeastern and the eastern Xifeng area was the mixed source area where the southwest, southeast, northeast and the east sources were convergent till the Chang 6 Member was deposited. The rare earth elements of the Chang 6 and Chang 8 Members are characterized by slight light rare earth-elements (LREE) enrichment and are slightly depleted in heavy rare earth-elements (HREE) with weak to moderate negative abnormal Eu, resulting in a right inclined REE pattern, which implies that the source rocks are closely related with better differential crust material. Analysis on geochemical characteristics of the mudstones and sandstones, features of parent rocks in provenance terranes and tectonic settings shows that source rocks for the Chang 8 Member mainly came from metamorphic and sedimentary rocks in transitional continental and basement uplift terranes with a small amount of rocks including metamorphic, sedimentary and igneous rocks coming from mixed recycle orogenic belt located in the southwest margin of the Ordos basin. Rocks in the crystalline basement and the overlying sedimentary cover in a basement uplift setting in the northeast periphery of the basin also contributed a part of the sources for the Chang 6 Member, in addition to the sources deriving from transitional continental and basement uplift terranes in the southwest margin of the basin. Parent rocks of the provenance terrane in the northeast margin of the Ordos Basin are characterized by having more felsic rocks.  相似文献   

10.
Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high‐grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote‐amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle‐derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted.  相似文献   

11.
A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments.

Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO2 rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO2 by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.  相似文献   


12.
Deformation of the Circum-Rhodope Belt Mesozoic (Middle Triassic to earliest Lower Cretaceous) low-grade schists underneath an arc-related ophiolitic magmatic suite and associated sedimentary successions in the eastern Rhodope-Thrace region occurred as a two-episode tectonic process: (i) Late Jurassic deformation of arc to margin units resulting from the eastern Rhodope-Evros arc–Rhodope terrane continental margin collision and accretion to that margin, and (ii) Middle Eocene deformation related to the Tertiary crustal extension and final collision resulting in the closure of the Vardar ocean south of the Rhodope terrane. The first deformational event D1 is expressed by Late Jurassic NW-N vergent fold generations and the main and subsidiary planar-linear structures. Although overprinting, these structural elements depict uniform bulk north-directed thrust kinematics and are geometrically compatible with the increments of progressive deformation that develops in same greenschist-facies metamorphic grade. It followed the Early-Middle Jurassic magmatic evolution of the eastern Rhodope-Evros arc established on the upper plate of the southward subducting Maliac-Meliata oceanic lithosphere that established the Vardar Ocean in a supra-subduction back-arc setting. This first event resulted in the thrust-related tectonic emplacement of the Mesozoic schists in a supra-crustal level onto the Rhodope continental margin. This Late Jurassic-Early Cretaceous tectonic event related to N-vergent Balkan orogeny is well-constrained by geochronological data and traced at a regional-scale within distinct units of the Carpatho-Balkan Belt. Following subduction reversal towards the north whereby the Vardar Ocean was subducted beneath the Rhodope margin by latest Cretaceous times, the low-grade schists aquired a new position in the upper plate, and hence, the Mesozoic schists are lacking the Cretaceous S-directed tectono-metamorphic episode whose effects are widespread in the underlying high-grade basement. The subduction of the remnant Vardar Ocean located behind the colliding arc since the middle Cretaceous was responsible for its ultimate closure, Early Tertiary collision with the Pelagonian block and extension in the region caused the extensional collapse related to the second deformational event D2. This extensional episode was experienced passively by the Mesozoic schists located in the hanging wall of the extensional detachments in Eocene times. It resulted in NE-SW oriented open folds representing corrugation antiforms of the extensional detachment surfaces, brittle faulting and burial history beneath thick Eocene sediments as indicated by 42.1–39.7 Ma 40Ar/39Ar mica plateau ages obtained in the study. The results provide structural constraints for the involvement components of Jurassic paleo-subduction zone in a Late Jurassic arc-continental margin collisional history that contributed to accretion-related crustal growth of the Rhodope terrane.  相似文献   

13.
The protocratonic core of the São Francisco craton assembled during the 2.1–2.0 Ga Transamazonian orogeny. Orosirian Fe‐rich sequences that extend from the northwestern border of the São Francisco protocraton (Colomi Group) to the southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group) record the opening of an intracratonic basin with the episodically developed ferruginous waters prior to the initiation of the Espinhaço rift at 1.8 Ga. Ferruginous conditions developed again during deposition of the Canjica Iron Formation of the < 1.7 Ga Serra de São José Group in the Espinhaço rift (contemporaneously with felsic magmatism; Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and extensive sandstones of the < (1666 ±32) Ma Itapanhoacanga and < (1683 ±11) Ma São João da Chapada Formations. In the upper São João da Chapada Formation, banded hematitic phyllite also records input of Fe‐rich fluids. The young age of these iron formations with respect to the conventionally accepted 1.88 Ga age for the youngest shallow‐marine Paleoproterozoic iron formations, the apparent absence of granular facies (granular iron formations), and yet shallow‐water (above fair‐weather base) depositional environment indicate that an unusual setting developed in a large basin after the Great Oxidation Event, in the aftermath of the Transamazonian orogeny. We propose that mantle plumes led to the opening of a previously unrecognized rift system, that could have caused the magmatism, supplied hydrothermal Fe and led to the opening of the Espinhaço, Pirapora, and Paramirim rifts, later obliterated by the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano orogeny. The rift system did not develop into an open continental margin but probably evolved into a broad sag basin, stretching across the São Francisco and Congo cratons.  相似文献   

14.
本文利用奥陶系米钵山组砂岩地球化学分析,结合区域地质研究,探讨贺兰山中、晚奥陶世的构造环境。贺兰山中段奥陶系米钵山组砂岩的地球化学研究表明,砂岩的siO2平均含量为81．3％;A120。／Si02值0．07～0．11,平均值为0．08;K20／Na20值变化较大,最大60．7,一般介于4．79～7．81;Fe2O3＋MgO含量较低,介于2．1％～2．81％。砂岩微量元素Nb丰度及V／（V＋Ni）与Ce／La、Sr／Ba值均较高,说明砂岩沉积于湿热、还原、低盐度环境,具有大陆型沉积特征。砂岩稀土元素富集,含量在116×10^-6～195×10^-6之间,平均值为158×10～;8Eu为0．52～0．58,具显著的负铕异常。这些数据指示了米钵山组具有重力流快速堆积的特征和大量陆源补给,浊流沉积作用是重力流携带陆源物质的主要途径。通过多种构造环境判别图解分析,显示物源区地质构造具有被动大陆边缘性质。  相似文献   

15.
During the geological survey of the metamorphic rocks in Xingning-Wuhua region on the western side of Wuyi Mountain, South China, we discovered the Neoproterozoic rhyolite and rhyolitic greywacke for the first time that outcrop in the Proterozoic metamorphic rocks near Jingnan Town of Xingning County, eastern Guangdong Province. A systematic research on petrology, geochemistry and geochronology of rhyolitic rocks was conducted to understand their tectonic setting and formation age. The Jingnan rhyolite is interbedded with a coeval greywacke, with a total thickness of 60 m; both rhyolite and greywacke display a similar folding and metamorphic pattern. Meta-rhyolite consists of groundmass and phenocrystals including sanidine, orthoclase, and quartz with distinct undulose extinction; the groundmass has been recrystallized into fine-grain feldspar, quartz and sericite aggregation. Meta-greywacke is composed of crystallinoclastic grains (sanidine, orthoclase, quartz and oligoclase) and clay groundmass. Zircon grains used for the SHRIMP U-Pb analysis are light brown-colored and euhedral or subeuhedral. Dating data suggest two age groups; eight grains of magmatype zircon with an idiomorphic form yield an age of 972±8 Ma, and the other seven weakly corroded grains of zircon with euhedral to subeuhedral shape construct an average age of 1097±11 Ma, which were captured from older rocks by an uplifting magma, implying that a late Mesoproterozoic basement exists in the Nanling region. In addition, one Paleoproterozoic age, 2035±11 Ma, is obtained from a rounded detrital zircon, indicating that a Paleoproterozoic thermal event took place in the South China. Geochemically, the Jingnan rhyolitic rocks are characterized by high K2O content, intermediate Al2O3 content, with the ACNK value 0.98―1.11, and belonging to high-K alkaline series. They are rich in ΣREE, Rb, Th and Ce, depleted in Ba, Sr, Eu, Ti, P and Nb-Ta, and with moderate negative Eu and Sr anomalies. These features indicate that the Jingnan volcanic rocks have an affinity of continental arc that is similar to those of acid volcanic rocks in the SE-China Coastal Region, in other words, a Neoproterozoic tectonomagma event might have taken place in the western Wuyi region, leading to an eruption of high-K calc-alkaline granitic magma.  相似文献   

16.
The Upper Triassic Langjiexue Group in southeastern Tibet has long been an enigmatic geological unit. It belongs tectonically to the northern Tethys Himalayan zone, but provenance signatures of the detritus it contains are significantly different from those of typical Tethys Himalayan sandstones. Because the Langjiexue Group is everywhere in fault contact with Tethys Himalayan strata, its original paleogeographic position has remained controversial for a long time. According to some researchers, the Langjiexue Group was deposited onto the northern edge of the Indian passive continental margin, whereas others interpreted it as an independent block accreted to the northern Indian margin only during final India-Asia convergence and collision in the Paleocene. This study compares the Langjiexue Group and coeval Upper Triassic strata of the southern Tethys Himalayan zone(Qulonggongba Formation). Our new provenance data indicate that Qulonggongba Formation sandstones contain common felsic volcanic rock fragments, minor plagioclase, and euhedral to subhedral zircon grains yielding Late Paleozoic to Triassic ages. These provenance features compare well with those of the Langjiexue Group. Because the Qulonggongba Formation certainly belongs to the Tethys Himalayan zone, the provenance similarity with the Langjiexue Group indicates that the latter is also an in situ Tethys Himalayan sedimentary sequence rather than part of an exotic block. Volcanic detritus including Late Paleozoic to Triassic zircon grains in both Langjiexue Group and Qulonggongba Formation are interpreted to have been derived from the distant Gondwanide orogen generated by Pan-Pacific subduction beneath the southeastern margin of Gondwana. The Qulonggongba Formation, deposited above marlstones of the lower Upper Triassic Tulong Group, is overlain by India-derived coastal quartzose sandstones of the uppermost Triassic Derirong Formation. Deposition of both the Qulonggongba Formation and the Langjiexue Group were most likely controlled by regional tectonism, possibly a rifting event along the northern margin of Gondwana.  相似文献   

17.
Edwin  Ortiz  Barry P.  Roser 《Island Arc》2006,15(2):223-238
Abstract Basement rocks in the catchment of the Kando River in southwest Japan can be divided into two main groups. Paleogene to Cretaceous felsic granitoids and volcanic rocks dominate in the upstream section, and more mafic, mostly Miocene volcanic and volcaniclastic rocks occur in the downstream reaches. Geochemically distinctive Mount Sambe adakitic volcanic products also crop out in the west. X‐ray fluorescence analyses of major elements and 14 trace elements were made of two size fractions (<180 and 180–2000 µm) from 86 stream sediments collected within the catchment, to examine contrasts in composition between the fractions as a result of sorting and varying source lithotype. The <180 µm fractions are depleted in SiO2 and enriched in most other major and trace elements relative to the 180–2000 µm fractions. Na2O, K2O, Ba, Rb and Sr are either depleted relative to the 180–2000 µm fractions, or show little contrast in abundance. Sediments from granitoid‐dominated catchments are distinguished by greater K2O, Th, Rb, Ba and Nb than those derived from the Miocene volcanic rocks. Granitoid‐derived <180 µm fractions are also enriched in Zr, Ce and Y. Sediments derived from the Miocene volcanic rocks generally contain greater TiO2, Fe2O3*, Sc, V, MgO and P2O5, reflecting their more mafic source. Sediments containing Sambe volcanic rocks in their source are marked by higher Sr, CaO, Na2O and lower Y, reflecting an adakitic signature that persists into the lower main channel, where compositions become less variable as the bedload is homogenized. Normalization against source averages shows that compositions of the 180–2000 µm fractions are less fractionated from their parents than are the <180 µm fractions, which are enriched for some elements. Contrast between the size fractions is greatest for the granitoid‐derived sediments. Weathering indices of the sediments are relatively low, indicating source weathering is moderate, and typical of temperate climates. Some zircon concentration has occurred in granitoid‐derived <180 µm fractions relative to 180–2000 µm counterparts, but Th/Sc and Zr/Sc ratios overall closely reflect both provenance and homogenization in the lower reaches.  相似文献   

18.

The geodynamic setting of the Bikou volcanic group is a critical question to trace the Precambrain tectonic framework and evolution for the Yangtze plate. This study has suggested that the Bikou volcanic group is composed of several residual oceanic crust units: MORB (mid-ocean ridge basalt), Alk-OIB (alkaline ocean island basalt) and Th-OIB (tholeiitic ocean island basalt) as well as subduction-related volcanic rocks. According to field observation, those distinct rocks occurred collectively in form of tectonic contact, implying that the Bikou volcanic group was an ophiolitic mélange. Coupled with geochronological data, a perished oceanic basin at the northern margin of the Yangtze block during Neoproterozoic was tested by this ophiolitic mélange. Meanwhile, the isogeochemical data suggest that the ocean occurred in the Southern Hemisphere identical to Indian, South Atlantic and South Pacific oceans in terms of their Dupal anomalies, and the original source of the rocks could be probably mixing by EMI and EMII component caused by dehydration melting of subducting oceanic crust during subduction process. On the basis of geochemical characteristics of the studied rocks, the Bikou volcanic group could imply that a partial breakup event occurred in the northern margin of Yangtze plate during the Neoproterozoic era.

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19.
This study evaluated the Cretaceous(Campanian–Maastrichtian) kaolinitic sediments of the Ajali/Mamu and Enugu/Nkporo Formations from the Lower Benue Trough of Nigeria. A combined method of inductively coupled plasma–mass spectrometry and isotope ratio mass spectrometry was used to investigate trace and rareearth element geochemistry and hydrogen and oxygen isotopic compositions. These data were then used to infer the sediments' provenance and paleoclimatic conditions during their deposition. The sediments contained low concentrations of most trace elements, with the exceptions of Zr(651–1352 ppm), Ba(56–157 ppm), V(38–90 ppm),and Sr(15.1–59.6 ppm). Average values of Co and Ni were1.5 and 0.7 ppm, respectively. Trace and rare earth element values were lower than corresponding values for upper continental crust and Post-Archean Australian Shale, with the exception of Zr. The samples showed only slight light rare-earth enrichment and nearly flat heavy rare-earth depletion patterns, with negative Eu and Tm anomalies,typical of felsic sources. Geochemical parameters such as La/Sc, Th/Sc, and Th/Co ratios support that the kaolinitic sediments were derived from a felsic rock source, likely deposited in an oxic environment.^(18 )O values ranged from+ 15.4 to + 21.2% for the investigated samples, consistent with a residual material derived from chemicalweathering of felsic rock and redeposited in a sedimentary basin(typical values of + 19 to + 21.2%). While in the basin, the sediments experienced extended interactions with meteoric water enriched in d D and d16 O. However,the variation in d D and d16 O values for the investigated samples is attributed to the high temperature of formation(54–91 °C). The d D and d^(18 )O values suggest that the sediments, although obtained from different localities within the Lower Benue Trough, formed under similar hot,tropical climatic conditions.  相似文献   

20.
The samples from the hidden Daqiling muscovite monzonite granite, which has recently been recognized within the Limu Sn-polymetallic ore field, have been analyzed for zircon U-Pb ages and whole rock geochemical and Nd-Hf isotopic compositions to discuss its genesis, source, and tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the granite crystallized in the late Indosinian (224.8±1.6 Ma). The granite is enriched in SiO2 and K2O and low in CaO and Na2O. It is strongly peraluminous with the A/CNK values of 1.09–1.20 and 1.4 vol%–2.7 vol% normal corundum. Chondrite-normalized REE patterns show slightly right-dipping shape with strongly negative Eu anomalies (δEu =0.08–0.17). All samples show enrichment of LILEs (Cs, Rb and K) and HFSEs (U, Pb, Ce and Hf), but have relatively low contents of Ba, Sr and Ti. The zircon saturation temperatures (T zr) are from 711 to 740°C, which are slightly lower than the average value of typical S-type granite (764°C). The granite has negative ? Nd(t) and ? Hf(t) values, which change from ?9.1 to ?10.1 with the peak values of ?9.2 to ?9.0 and from ?3.7 to ?12.6 with the peak values of ?6 to ?5, respectively. The T DM C (Nd) and T DM C (Hf) values are 1.74–1.82 Ga with the peak values of 1.73–1.75 Ga and 1.49–2.04 Ga with the peak values of 1.5–1.6 Ga, respectively. These characteristics reveal that the source region of the granite is dominantly late Paleoproterozoic to early Mesoproterozoic crustal materials. Seven inherited magmatic zircons are dated at the age of 248.6±4.3 Ma, which suggests the existence of the early Indosinian granite in Limu area. These zircons have the ? Hf(t) values of ?6.7–?2.3, similar to those of the Daqiling granite, implying the involvement of the early Indosinian granite during the formation of the Daqiling granite. Inherited zircon of 945±11 Ma has the ? Hf(t) and T DM(Hf) values of 8.7 and 1.14 Ga, respectively, compatible with those of the Neoproterozoic arc magmatic rocks in the eastern Jiangnan orogenic belt. Therefore we inferred that Neoproterozoic arc magma might have been involved in the formation of the Daqiling granite, and that the Neoproterozoic arc magma belt and continent-arc collision belt between the Yangtze and Cathaysia Blocks might have extended westsouthward to Limu region. It is proposed that the underplating of mantle materials triggered by crustal extension and thinning resulted in partial melting of crustal materials to form the Daqiling granite in the late Indosinian under post-collisional tectonic setting.  相似文献   

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