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1.

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A’nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur’ngoi diorite in the Kuhai-A’nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur’ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur’ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A’nyêmaqên suggests that the southern margin of the “Qilian-Qaidam-Kunlun” archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.

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2.
The study of nearly 100 new trace element analyses of late Cambrian/early Ordovician greenstones from the Grong, Løkken, Støren and Stavenes areas of central and southern Norway indicates the presence of ocean floor type basalts in all four areas, and the presence of low-potassium tholeiites of island arc affinity in at least two of the areas. From this result an ocean floor-based Ordovician/early Silurian island arc complex and back-arc eugeosynclinal pile is recognised in the Trondheim region and its principal features described. This complex is considered to have been obducted during an early stage of the Middle Silurian orogeny upon an Eocambrian/Ordovician miogeosynclinal sedimentary succession which had developed on continental crust. During a later stage of the major folding and metamorphism of the juxtaposed sequences, further eastward thrusting took place such that total displacement of some allochthonous sheets may be in the order of several hundred kilometres.  相似文献   

3.
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   

4.
Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO2 and Na2O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al2O3, and depleted in TiO2, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg? and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.  相似文献   

5.
Abstract Geochemical analyses of volcanic rocks in the Gamilaroi terrane reveal several phases of arc activity within an intra-oceanic island-arc terrane. Felsic volcanic rocks at the base of the section have rare earth element (REE) and trace element compositions which indicate that they were derived from an island-arc source. Basalts immediately overlying the felsic volcanic rocks have a distinctive geochemical signature with low levels of Ti and Y and high levels of Ni, Cr and Mg. Low concentrations of REE and trace elements relative to mid-ocean-ridge basalts (MORB) indicate that they were also derived from an intra-oceanic island-arc source. Extensive basalts and basaltic andesites among the youngest rocks of the terrane have typically flat to enriched REE and trace element compositions, indicating a transitional arc-back-arc source. The change in basalt compositions indicates that rifting had occurred by this stage in the evolution of the arc. Confirmation of an intra-oceanic setting for this terrane enables a more detailed comparison with similar intra-oceanic rocks in the northern New England orogen. This study of the Gamilaroi terrane is an example of the potential use of geochemical data to identify other ancient intra-oceanic island-arc-rift suites.  相似文献   

6.
Granodiorite and diorite plutons of the Rosetown complex, N.Y., which are associated with the nearby Cortlandt complex, have chemical and textural characteristics indicating that large-scale liquid immiscibility played a major role in their petrogenesis. Rare earth element, zirconium, niobium and phosphorus abundances are much greater in the diorite precluding the possibility that the Rosetown diorite and granodiorite are related by fractional crystallization. The trace element data also eliminate the possibility that the granodiorite represents: (1) a partial melt of crustal rocks including basalt; (2) a granitic cumulate; or (3) a residue from an aqueous fluid derived either from a silicate melt or crustal rocks. Liquid immiscibility appears to be viable model for the origin of the Rosetown granodiorite and iron-rich diorite. This model is supported by the following: (1) the major element compositions occur in a two-liquid field on a Greig diagram; (2) both bodies have similar Sr isotope compositions; (3) common phases in the two rock types have overlapping compositions; (4) the major and trace element data of the diorite and granodiorite are similar to the experimentally determined partition data of immiscible liquid pairs; and (5) possible ocelli of iron-rich diorite are found in the granodiorite.  相似文献   

7.
The subduction of “hot” Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back-arc and island-arc sides of southwest Japan. The Goto Islands are located on the back-arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori-jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U–Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U–Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at ~16–15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15–14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.  相似文献   

8.
In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28°N, 14°E (k = 25, α95 = 8.8°, N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but can help to constrain the apparent polar wander path for Gondwana. Our results give only paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60–70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval.  相似文献   

9.
The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.  相似文献   

10.
湘西南兰蓉岩体为一加里东期小侵入体,由黑云母二长花岗岩和二云母二长花岗岩组成.（443.5±8.1）Ma的锆石SHRIMP U Pb年龄表明花岗岩形成于早志留世早期.主量元素组成表明岩体总体属钙碱性高钾钙碱性系列强过铝质花岗岩类.该侵入体Ba、（Ta＋Nb）、Sr、P、Ti强烈亏损,Rb、（Th＋U＋K）、（La＋Ce）、Nd、（Zr＋Hf＋Sm）、（Y＋Yb＋Lu）等相对富集;稀土元素含量较高、轻稀土富集明显、Eu显著亏损;Isr值为0.71299,εSr（t）值为120,εNd （t）值为 8.11和-8.89,t2DM为1.82和1.84Ga.C/MF-A/MF图解显示其源岩为泥质岩和砂屑岩.上述地球化学特征表明兰蓉岩体为陆壳碎屑岩石部分熔融形成的S型花岗岩.基于岩石成因、构造环境判别以及区域构造演化过程,推断兰蓉岩体的具体形成机制为：奥陶纪末志留纪初的北流运动（板内造山运动）导致地壳增厚、升温,尔后在挤压减弱、应力松弛的后碰撞减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位而形成兰蓉岩体.  相似文献   

11.
Abundant metabasites occur in highly deformed granitic and migmatitic gneisses as blocks and lenses of tens of meter size around the Haiyangsuo area, northeast part of Sulu UHP belt, eastern China. They comprise garnet-pyroxene granulites, eclogitized granulites and amphibolites. Their protolith compositions were mainly olivine tholeiite and quartz tholeiite, and show variation from Mg-rich to Fe-rich component as tholeiitic cumulates. Pearce’s element ratio slopes suggested that protolith of these rocks were comagmatic, and generated from a primary magma by fractional crystallization of plagioclase, olivine and clinopyroxene. The crystallization differentiation has also been evidenced by trace elements, such as parallel REE patterns, Ni vs Ce variations, Sr increasing depletions, although the large ion lithophile elements (LILE) were modified to different extent during metamorphism. Trace element composition and Nd isotopes indicate a depleted mantle origin for these rocks. But they are not likely to be fragments of ophiolites or tholeiites connected with subduction, they formed probably at intra-continent environment. Sm-Nd whole rock isochron age of 2252±180Ma indicates approximately the formation age of igneous protolith of these rocks, almost 2000Ma earlier than the formation of the Dabie-Sulu UHP collision zone at about 240–220 Ma.  相似文献   

12.
Quanshu Yan  Xuefa Shi 《Island Arc》2014,23(3):221-235
Major element and trace element compositions, and Sr, Nd and Pb isotopic compositions for postcollisional granites from the Laoshan granitic complex, in the eastern side of the Triassic suture between the South China and North China tectonic blocks were determined. The granites are alkaline, A‐type and can be further classified as A1 granites. The trace element composition of these granites is transitional between those of oceanic island basalt and enriched mid‐oceanic ridge basalt, with depletions in Ba, Sr, P, and Ti that can be ascribed to mineral fractionation and enrichments in Cs, Rb, Th and U possibly resulted from the involvement of slab fluids. The isotopic signature of Laoshan granites represent a mixture between an enriched mantle type 1 (EMI)‐like end‐member and lower continental crust (LCC). We propose that the magmas that formed the Laoshan A1 granites are a mixture between those derived from the EMI‐like delaminated eclogitic rocks (subsequently enriched by fluids released from Mesozoic Pacific subducted slab) and those derived from the LCC, which consists of granulites or metamorphic residues from the prior generation of I‐type granites in the region. The mixed magmas then experienced a strongly alkali feldspar‐dominated fractionation prior to their emplacements as A‐type granites in the Laoshan granitic complex.  相似文献   

13.
Ichnosedimentologic evidence of periodic anoxic shelf in the Early-Middle Ordovician transition includes lower ichnodiversity, shallower bioturbation and burrowing depth (<4 cm), rare domichnia, tinyChondrites occupying shallower or shallowest tiering, widely distributed nodules of limonite pseudomorphs after pyrite, occurrence of trace fossils being closely associated with the storm event layers, and stratigraphic successions with orbital cyclostratigraphic architecture. It is suggested that lower atmospheric oxygen level during the Early Paleozoic, the Ordovician radiation, dramatic transgression and warmer temperatures would result in the periodic anoxia in the Early-Middle Ordovician transition. This episode began at the later Early Ordovician and lasted about 3.4 Ma on the basis of orbital cyclostratigraphy.  相似文献   

14.
The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.  相似文献   

15.
Liu  Yican  Xu  Shutong  Li  Shuguang  Jiang  Laili  Wu  Weiping  Chen  Guanbao  Su  Wen 《中国科学:地球科学(英文版)》2000,43(1):178-188

The petrologic geochemical and Sr−Nd isotopic compositions of the eclogites from the mafic-ultramafic rock belt (MUMRB) in the Northern Dabie Mountains indicate that: (1) the protoliths of most of eclogites are tholeitic basalt and a few may be gabbro, and most of them produced from the Yangtze subducted continental crust (lower crust and formed during the deep subduction) and a part may be from paleo-oceanic relics between the Yangtze and North China continental plates; (2) their positive Nb anomalies and related trace element characteristics show that they did not form in the island-arc setting; (3) the metamorphosed MUMRB with eclogite and meta-peridotite blocks along the southern part of the Mozitan-Xiaotian fault zone may represent the suture zone produced during the collision between the Yangtze and North China continental plates, which included the Yangtze subducted continental crust and paleo-oceanic relics.

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16.
Palaeomagnetic results from the Lower Palaeozoic inliers of northern England cover the upper part of the (Middle Ordovician) Borrowdale Volcanic Series (palaeomagnetic pole 208°E, 18°S, A95 = 9.4°), minor extrusive units relating to the Caradoc and Ashgill stages of Ordovician times, intrusive episodes of Middle Ordovician and Middle Silurian to Late Devonian age, and the Shap Granite of Devonian (393 m.y.) age (palaeomagnetic pole 313°E, 33°S, A95 = 5.6°).A complete assessment of Ordovician to Devonian palaeomagnetic data for the British region shows that the pole was nearly static relative to this region for long intervals which were separated by shifts occupying no more than a few millions of years. The mean palaeomagnetic poles are: Ordovician (6°E, 16°S), Lower Silurian (58°E, 16°N), Middle Silurian/Lower Devonian (318°E, 6°N) and Middle/Upper Devonian (338°E, 26°S); the first two shifts separating these mean poles can be explained predominantly in terms of rotational movements of the crustal plate but the last involved appreciable movement in palaeolatitude.Comparison of Lower Palaeozoic palaeomagnetic data from the British region with contemporaneous data from continental Europe/North America on the Pangaean reconstruction reveals a systematic discrepancy in palaeolatitude between the two regions prior to Middle Devonian times. This discrepancy was eliminated during a few millions of years of Lower/Middle Devonian times (ca. 395 m.y.) and can be explained in terms of ca. 3500 km of sinistral strike-slip movement close to the line of the orthotectonic Caledonides. This motion is linked both in time and place to the impingement of the Gondwanaland and Laurentian supercontinents during the Acadian orogeny; this appears to have displaced the British sub-plate until it became effectively locked between the Baltic and Laurentian regions. Although movement of the dipole field relative to the British region in Lower Palaeozoic times is now well defined, nearly one fifth of the total data show that the geomagnetic field was more complex than dipolar during this interval. Until the significance of these anomalies is fully resolved, the tectonic model derived from the palaeomagnetic data cannot be regarded as unambiguous.  相似文献   

17.
Dong  YunPeng  Zhang  GuoWei  Yang  Zhao  Zhao  Xia  Ma  HaiYong  Yao  AnPing 《中国科学:地球科学(英文版)》2007,50(2):234-245

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO2 and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO2 content, low Al2O3 content and Na2O>>K2O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geo-chronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.

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18.

Many light rare earth deposits, such as Maoniuping, Dalucao, Panzhihua deposits, are collectively distributed in Panxi rift of Sichuan Province, China, and closely associated with the aegirine quartz syenite-carbonatite complex. Carbon and oxygen isotope studies demonstrate that the carbonatites in the complex are of typical igneous origin related to mantle processes. Electronic microprobe studies show that the fluid-melt inclusions found in the complex are enriched in light rare earth elements (LREE), which suggests that the magma was rich in LREE and could serve as the ore source for the regional LREE mineralization. Both the aegirine quartz syenite-carbonatite complex and the LREE mineralization found therein were derived from the mantle. The rare gas isotope analyses also support that there is a genetic association between the LREE mineralization and mantle processes.

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19.
The Late Mesozoic-Cenozoic volcanism of the Tugnui-Khilok sector in the western Transbaikalia rift area is related to the development of the Tugnui, Tsolgin, Margentui, and Khilok grabens and is characterized by a north-south migration of magmatic centers. In these grabens, the igneous associations are composed of high-alkaline rocks: alkaline and subalkaline basalts, tephrites, phonolites, trachytes, trachyrhyolites, comendites and pantellerites, alkaline syenites and alkaline gabbroids. These associations are known to have formed during 10 stages: Late Jurassic (150–158 Ma), Late Jurassic-Early Cretaceous (139–147 Ma), the beginning of the Early Cretaceous (133–145 Ma), mid-Early Cretaceous (115–134Ma), the end of the Early Cretaceous (104–114 Ma), the end of the Early-beginning of Late Cretaceous (99–102 Ma), Late Cretaceous (72–90 Ma), Eocene (38–48 Ma), Early Oligocene (30–35 Ma), and Late Oligocene (25–27 Ma). The composition of igneous associations was changing in such a way that the relative amount of salic rocks gradually decreased (occasionally even disappeared completely) in the later developmental stages. As well, the content of SiO2 in basic rocks also decreased with increasing Nb and Ta contents, and depletion occurred in the lithophylic elements Rb, K, Ba, Sr, and in light rare-earths relative to heavy ones. The geochemical and isotope-geochemical parameters of basaltoids change through time, probably due to successive changes in the mantle sources of magmatism. During Mesozoic time, the source composition was consistent, with OIB-EM-II sources enriched in radiogenic strontium, but since the second half of the Cretaceous, the isotope composition began to be modified toward moderately depleted sources of the OIB-PREMA type.  相似文献   

20.
Abstract   Small-volume plutons of Early to Late Cretaceous ages are widely distributed in the Yamizo Mountains, central Japan. These plutons consist predominantly of granitoids, classified into hornblende gabbro, quartz diorite, hornblende–biotite granodiorite and coarse-grained biotite granite. The quartz diorite (52–64 wt% of SiO2) is characterized by a high Sr content (606–769 p.p.m.) associated with a low Y (13–27 p.p.m.) and heavy rare earth element content (Yb content of 1.19–2.13 p.p.m.). On the Sr/Y versus Y diagram, this rock type mainly plots in the adakite and Archean high-Al tonalite, trondhjemite and granodiorite (TTG) field. Together with its initial Sr isotopic ratios, which range from 0.7038 to 0.7046, these data suggest that quartz diorite originated as slab melts. However, geochemical calculations assuming either eclogite or garnet amphibolite as the source material do not support this suggestion. Instead, the chemical compositions of quartz diorite are better explained by the fractional crystallization of hornblende, plagioclase and biotite from a primitive, basaltic melt in a magma chamber. In this case, the formation of the associated hornblende gabbro can also be explained by the accumulation of hornblende and plagioclase. Adakitic rocks of Early Cretaceous ages have also been reported in the Tamba Belt of the inner zone of southwest Japan, located ca 500 km west of the Yamizo Mountains. These rocks can be correlated to the adakitic rocks in the Yamizo Mountains based on the geology, petrography, geochemistry and radiometric ages. Therefore, we propose the possibility that the Early Cretaceous adakitic rocks in the inner zone of southwest Japan were produced by fractional crystallization from basaltic arc magmas generated by a partial melting of metasomatized wedge mantle peridotite.  相似文献   

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