Ages, Sources and Tectonic Settings of the Triassic Igneous Rocks in the Easternmost Segment of the East Kunlun Orogen, Central China

U–Pb analysis of zircons from igneous rocks in the Elashan Mountain, easternmost segment of the East Kunlun Orogen yielded 252–232 Ma. Geochemically, these rocks are mainly high in SiO_2, K_2O and K_2O＋Na_2O contents, low in P_2O_5 and TiO_2 contents, depleted in Ba, Sr, P, Ti and enriched in U, Hf, Zr, showing features of I–type granite. The zircon εHf（t） values of the Early Triassic Jiamuge＇er rhyolite porphyry（252±3 Ma） are positive（＋1.6 to ＋12.1）, suggesting a juvenile crustal source mixing with little old crustal component, and the zircon εHf（t） values of the Middle Triassic Manzhang＇gang granodiorite（244±3 Ma） and Dehailong diorite（237±3 Ma） are predominately negative（-8.4 to ＋1.0）, indicating an older crustal source. In comparison, the zircon εHf（t） values of the Late Triassic syenogranites from Suigen＇ergang（234±2Ma）, Ge＇ermugang（233±2 Ma） and Yue＇ergen（232±3 Ma） plutons vary from-3.8 to ＋5.0, suggesting a crust-mantle mixing source. From Early–Middle Triassic（252–237 Ma） to Late Triassic（234–232 Ma）, the geochemical characteristics of these rocks show the change from a subduction–collision setting to a post-collision or within-plate setting. By comparing of these new age data with 77 zircon U–Pb ages of igneous rocks of the eastern part of East Kunlun orogen from published literatures, we conclude that the igneous rocks of Elashan Mountain and these of the eastern part of East Kunlun Orogen belong to one magmatic belt. All these data indicate that the Triassic magmatic events of the eastern part of East Kunlun Orogen can be divided into three stages： 252–238 Ma, 238–226 Ma and 226–212 Ma. Statistically, the average εHf（t） values of the threestage igneous rocks show a tendency, from the old to young, from-0.75±0.25 to lower-2.65±0.52 and then to-1.22±0.25, respectively, which reveal the change of their sources. These characteristics can be explained as a crust-mantle mixing source generated in a subductional stage, mainly crust source in a syn–collisional stage and a crust-mantle mixing source（lower crust with mantle-derived underplating magma） in a post-collisional stage. The identification of these three magmatic events in the Elashan Mountain, including all the eastern part of East Kunlun Orogen, provides new evidence for better understanding of the tectonic evolution of the northward subduction and closure of the Paleo-Tethyan（252–238 Ma）, the collision of the Songpan–Ganzi block with the southern margin of Qaidam block（238–226 Ma）, and the post–collisional setting（226–212 Ma） during the Early Mesozoic period.

Paleo-Tethyan Oceanic Crust Subduction in the Eastern Section of the East Kunlun Orogenic Belt:Geochronology and Petrogenesis of the Qushi'ang Granodiorite

The Qushi’ang granodiorite（QSG） is located at the central east of the ophiolitic melange belt in the East Kunlun Orogenic Belt（EKOB） in the northern margin of the Qinghai-Tibetan Plateau. LA-MC-ICP-MS zircon U–Pb dating suggests that the granodiorite and mafic microgranular enclaves（MMEs） crystallized 246.61±0.62 and 245.45±0.9 Ma ago, respectively. Granodiorite, porphyritic diorite, and MMEs are metaluminous and medium-K calk-alkaline series, with island-arc magma features, such as LILE enrichment and HFSE depletion. The porphyritic diorite has high Cr（13.50 ppm to 59.01 ppm）, Ni（228.53 ppm to 261.29 ppm）, and Mg~#（46–54）. Granodiorite and porphyritic diorite have similar mineral compositions and evolved major and trace elements contents, particularly Cr and Ni, both of which are significantly higher than that in granites of the same period. The crystallization age of MMEs is close to that of granodiorite, and their major and trace elements contents are in-between porphyritic diorite and granodiorite. The results suggest that the original mafic magma, which was the product of mantle melting by subduction process, intruded into the lower crust（Kuhai Rock Group）, resulting in the formation of granodiorite. Countinous intrusion of mafic magma into the unconsolidated granodiorite formed MMEs and porphyritic diorite. The granodiorite reformed by late-stage strike-slip faulting tectonic event indicates that the strike-slip fault of Middle Kunlun and the collision of the Bayanhar block with East Kunlun were later than 246 Ma. Therefore, the formation of the QSG not only indicates the critical period of evolution of East Kunlun but also represents the tectonic transition from oceanic crust subduction to slab breaking.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt,NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare.In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun.Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet,orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 ℃and 5.6–7.0 kbar,representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma,413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with(La/Sm)Nvalues of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high(^(87)Sr/^(86)Sr)_(i)values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values(-4.3 to 2.5) and variable whole-rock εNd(t) values(-4.9 to 1.2) as well as high(^(87)Sr/86 Sr)ivalues(0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from -30.8 to -5.1, while the whole-rock samples yield consistent(^(87)Sr/86 Sr)ivalues(0.7301 to 0.7342) and negative εNd(t) values(-10.1 to -12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Late Cambrian SSZ-type Ophiolites in Acite Zone, East Kunlun Orogen of Northern Tibet Plateau: Insights from Zircon U-Pb Isotopes and Geochemistry of Oceanic Crust Rocks

1 Introduction East Kunlun orogen(EKO)stretching more than 1000km in E-W extension is located in the western segment of Central Orogen Belt(COB),China(Xu et al.,2006,Li et al.,2014).There outcropped Cambrian ophiolites

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

The East Kunlun Orogenic Belt(EKOB)in northeast margin of the Qinghai-Tibetan Plateau is an important part of the Central Orogenic System(COS).During the long-time geological evolution,complicated tectono

Cenozoic Exhumation History of the East Kunlun Orogenic Belt Constrained by Apatite Fission-Track Thermochronology

Objective The East Kunlun Orogenic belt constitutes the first marked change in the topographic reliefs north of the Qinghai-Tibet Plateau.The Cenozoic tectonic evolution of this orogenic belt is crucial for understanding the remote deformational effects of the Eurasian plate collision and the migration track at the northern margin of the plateau.However,when and how the uplift occurred remains

Magma Mixing and Mingling for Xiangjiananshan Granitic batholith at eastern area of the East Kunlun Orogenic Belt

The Changning Menglian belt is an important area of research on the evolution of the Paleo Tethys ocean structure,the belt can be solved such as the Changning Menglianbeltposition;sequencestratigraphy;sedimentary environment;nature and its tectonic evolution history and tectonic domain and Gut Tis relationship;therefore,the research on Chang Ning Menglian zone have a great significance to solve many problems of the Sanjiang fold belt in Tethys and Himalaya tectonic area.'Hot spring'is located in the west margin of the southern Changning Menglian belt,studying Yunnan Fengqing hot spring group'geological and petrology characteristics roundly and in depth,concluding the metamorphism and deformation characteristics,clarifying the metamorphism effect and its stages,understanding the association its combination with the Changning Menglian belt between,therefore it has the great significance to solve the geological evolution history in the Sanjiang area,especially the paleo Tethyan tectonic belt,as well as Gondwana and Eurasia boundaries and other major problem.Through collect and read the literature data,measurement of field section,geological investigation,research and Study on rock sheet indoor,rock composition test,electron probe testing system,summarize the geological characteristics and petrological characteristics of'hot springs group',and through the discussion of the geochemical characteristics of rocks,explore its rock assemblages,characteristics of original rock and analysis of metamorphism and deformation stages,to provide basic data for regional geological evolution.The study shows that the main lithology is biotite quartz schist,mica schist and epimetamorphic sandstone interspersed with a small amount of phyllonite,granulite,silicalite,carbonaceous slate and phyllitic cataclasite that contains some pressure breccia.The metamorphic mineral paragenetic assemblage of the representative rock is:M1 biotite(Bi)+plagioclase(Pl)+quartz(Q),and M2 muscovite(Mus)+quartz(Q).The protolith is felsic rock and sedimentary rock that belongs to argiloid.On the basis of comparison,the stratigraphic sequence of the protolith is consistent with the type section of Wenquan formation.Along with the subduction(Hercynian)-subduction(Indosinian)-orogenic(Yanshan Himalayan period)process of Changning Menglian belt,hot springs group experienced two stages of metamorphism and three stages of deformation,metamorphic temperature at400-500℃,the pressure is foucs on 0.3-0.62Gpa,and shown the retrograde metamorphism of the low greenschist facies.Geological age of hot springs formation is early Devonian(survey team of Yunnan District three units,1980),sedimentary environment is mainly shallow and semi deep sea,observed Bouma sequence in rock slice,therefore,the depositional environment may be fan or basin of sea,the sedimentary formations are mainly clastic rocksiliceous rock formation,the upper coal—contained formation.With the Changning Meng Lian ocean expansion,ocean island begin to develop,material deposition continuing,appearing volcano material,the protolith may contain volcano matter through studying the thin section.To the Late Permian,Crust of Changning Menglian ocean begin to subduct to the east of the Yangtze block,ocean basin began to close,but it still has formation here at this time,mainly shallow carbonate formation,with proceeding of subduction,in the low temperature groove(7Km deep),due to changes in temperature and pressurer,appearing metamorphism(M1)and deformation(D1)for the first time,the shear effect produced by deformation lead to some cleavage,occurring regional foliation S1,major metamorphic minerals formed in metamorphism is long flake biotite.The main metamorphic mineral assemblages are biotite(Bi)+feldspar(Pl)+quartz(Q).Subsequently,crustal uplift,depositional break,because the Changning Meng Lianyang has closed during the Indosinian period,Baoshan-Zhenkang block in the west and the Yangtze block in the east knocked each other.In the Indosinian,under the action of faults,the hot spring formation clipping and retracing,back to a position about1-2Km depth,the position is still belongs to the low temperature groove,and occurring axial cleavage in the core of the fold,namely S2.That is,the emergence of the second metamorphism(M2)and deformation(D2).The deformation is affected by the strong pressure,so the rock have dewatered,so the second metamorphic deformation process is affected by temperature(T),pressure(Ps)and fluid(C).The main metamorphic minerals in the second generation of metamorphism is Muscovite,while there have some of biotite formed in same period,find that the first phase of biotite parallel growth of rock slice,namely S1 parallel S2,and we can see incomplete metamorphism biotite,so the the Muscovite is formed by the first stage of metamorphism and metamorphic biotite.The main mineral of the second stage metamorphism is Muscovite(Mus)+quartz(Q) Then,the crust continues to rise,the sedimentary break continues.In the Jurassic Cretaceous start orogeny,namely Yanshan period intracontinental orogeny,occurred third deformation(D3),under extrusion shearing,S3 emergencing,after Yanshan intracontinental orogenic period,in Himalayan period there have large-scale nappe structure and differential uplift and faulting.So the third deformation(D3)strengthened,with weak metamorphism,sericite emergencing.

U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 +5.0/-6.0 Ma, and the other orthogneiss sample con- tains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 +12/?17 Ma and 516 ± 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 +10/?8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Pre- cambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.

^(40)Ar-^(39)Ar and U-Pb ages of metadiorite from the East Kunlun Orogenic Belt: Evidence for Early-Paleozoic magmatic zone and excess argon in amphibole minerals

Single-grain zircon U-Pb and amphibole 40Ar-39Ar dating have been conducted on a deformed and metamorphosed diorite in the East Kunlun Orogenic Belt, which intruded into the middle Proterozoic Kuhai Group exposed in the south of Xiangride region, Dulan County, NW Qinghai Province. The zircon gives a concordant U-Pb age of (446.5±9.1) Ma. The amphibole yields Ar plateau age of (488.0±1.2) Ma and an isochronal age of (488.9±5.6) Ma. Age results of both stepwise released Ar and conventional K-Ar analysis are remarkably higher than that of zircon U-Pb, suggesting that the amphibole contains excess argon and the amphibole plateau age cannot be taken as the timing of metamorphism or deformation. The zircon age is interpreted to be crystallization age of the diorite pluton, which suggests that an Early-Paleozoic magmatic zone indeed existed in the East Kunlun Orogenic Belt stretching along the region south to the Golmud, Normuhong and Xiangride.

GEOLOGICAL EVOLUTION AND OROGENY OF EAST KUNLUN TERRAIN ON THE NORTHERN QINGHAI—TIBET PLATEAU

The East Kunlun terrain is located on the northern Qinghai—Tibet plateau, composed of the East Kunlun Mountain and the Qaidam Basin and bounded by the Qilian terrain on the north and Bayanhar—Songpan Ganze terrain on the south. It is regarded as a composite orogenic belt characterized by having developed superimposed ductile tectonic regimes reflecting the collision orogeny during Early Paleozoic and Triassic periods. It has also experienced transformation from ductile to brittle deformation caused by the post orogeny, since Jurassic after the formation of the East Kunlun Mountain and the Qaidam Basin. A Paleozoic subduction complex zone was recently recognized along the north border of the East Kunlun terrain from Da Qaidam to Dulan (Xu,et al, 1 999). It is composed of ophiolite (of Early Paleozoic age?), tectonic melange and very high\|pressure metamorphic rocks with eclogite (\%p\%=2 2GPa, \%t\%=720℃) (Yang,et al.,1998) and garnet\|peridotite (\%p\%=2 5GPa, t =837℃). The Anyemaqin Triassic subduction complex zone trending in NWW\|SEE was developed along the eastern segment of the south border of the East Kunlun terrain. It is mainly composed of ultramafic and mafic rocks, pillow basalt, radiolaria\|bearing clastic rocks, tectonic melange and mylonite. The subduction complex zone contains a series of the southward overthrusting imbricated slices. Instead of this subduction complex zone, a 2 km\|wide sinistral strike\|slip ductile shear zone trending in E\|W was developed along the western segment. On the basis of macroscopic and microscopic studies on a series of structural sections, we divide the East Kunlun Mountain into four tectonic units as follows: (1) North Proterozoic Metamorphic Basement Zone; (2) South Early Paleozoic—Triassic Superimposed Fold Zone; (3) South Triassic Transpression Zone; (4) Anyemaqin Back\|Arc Decollement\|Thrust Zone.

Petrogenesis and tectonic implications of Late Triassic granitoids in the Alananshan,East Kunlun belt:evidence from geochemistry,geochronology,and zircon Hf isotopic compositions

The Qimantag in the East Kunlun Orogenic Belt has widespread Triassic magmatic rocks that have received scant attention,with an unresolved issue relating to its petrogenesis and geodynamics.In this paper,we used zircon U-Pb-Hf isotopes and whole-rock geochemistry to trace the petrogenesis and tectonic settings of the moyite and monzogranite from the Qimantag Alananshan,East Kunlun.The moyite and monzogranite are silicic(SiO_(2-)~69.9-76.41%),highly alkali(Na_(2)O+K_(2)O~7.29 to8.96 wt.%),with Mg^(#)about 10.4-30.34,indicative of a high-K calc-alkaline rock series.The rare earth element patterns diagram is right-leaning,with a negative Eu anomaly(δEu=0.31-0.68).They are enriched in Rb,K,and light rare earth elements but depleted in Nb,Ta,and Ti,with abundant amphibole,typical of I-type granites.U-Pb on zircon constrained the emplacement of the moyite at 223.9±2.6 Ma and monzogranite at 226.9±2.9 Ma.TheεHf(t)values range from-2.8 to+0.1 except for one outlier value of-7.0,corresponding two-stage model age of 1249-1437 Ma.Our combined geochemical and isotopic results indicate that the moyite and monzogranite were derived from partial melting of the lower thicken crust with the contribution from the older basement materials.These rocks formed in a post-collision setting that is transitional between compressive collision and extension orogeny.

Petrogenesis and tectonic implications of early Paleozoic granitoids in East Kunlun belt: Evidences from geochronology, geochemistry and isotopes

The East Kunlun Orogenic Belt(EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism.Here we report the petrology, bulk geochemistry, zircon Ue Pb dating and, Lue Hf and SreN d isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U-Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450 -430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their(^(87)Sr/^(86)Sr)ivalues range from 0.7059 to 0.7085, εNd(t) values from -1.6 to -6.0 and the zircon εHf(t) values show large variations from +9.1 to -8.6 with Hf model ages(T_(DM2)) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.

Zircon U-Pb chronology, geochemistry and Lu-Hf isotope constraints on genesis of monzonitic granite from Harizha area in eastern section of East Kunlun region

The Harizha area is located in the eastern section of East Kunlun orogenic belt. The petrology, the zircon LA-ICP-MS U-Pb chronology and the petrogeochemistry of the monzonitic granite in the area were studied. The results show that the formation age of the monzonitic granite in the Harizha area is 237.4±1.4 Ma, which belongs to Middle Triassic. The content of SiO_2 in the rock is 61.29%--63.30%,(K_2O+Na_2O) is 5.41%--5.84%, Al_2O_3 is 14.80%--15.99%, the Mg~# value is 46.33--48.33, and the aluminum saturation index A/CNK is 0.87--0.91. Therefore, it can be concluded that the rock belongs to the I-type granite of quasi-aluminous high potassium Ca-alkaline series. The total REE content of the rock is 118.82×10^(-6)--164.54×10^(-6), and the La/Yb ratios range from 7.77 to 10.13. Meanwhile, the rock does not show obvious Eu anomalies(δEu=0.61--0.75) and is characterized by enrichment of LREE and LILE such as Rb and K, relative depletion of HREE and HFSE such as Nb, Ti and Ta. Zircon Hf isotopic dating exhibits that the average ε_(Hf)(t) is-2.4 and the average of two-stage model age(t_(DM2)) is 1 417 Ma, indicating that the source rocks are mainly crustal materials. It can be concluded from research findings and regional geological structural analysis that the monzonitic granite in the Harizha area originated from partial melting of the Mesoproterozoic mafic lower crust in the later stage of the subduction of the Paleo-Tethys Ocean.

东昆仑大格勒农场南辉长岩锆石U-Pb年代学、地球化学特征及地质意义

东昆仑志留纪岩浆活动强烈,记录了东昆仑造山带原特提斯洋演化晚期大洋俯冲阶段与碰撞阶段转换地质过程。本次研究对东昆仑大格勒农场南志留纪基性侵入岩开展了详细的岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究,以确定其形成时代、岩石成因和构造环境。研究结果表明大格勒农场南辉长岩的结晶年龄为430.2±2.4 Ma,形成时代为中志留世。辉长岩SiO_(2)(51.40%~53.66%)含量较低,MgO(18.56%~22.16%)和FeO^(T)(7.54%~10.64%)含量高,Mg^(#)为79~82。微量元素富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有高Ni(276~973μg/g)、Cr(2152~3574μg/g)和Sc(35.5~53.1μg/g)含量,低Nb/Ta值(3.23~8.68)和εHf(t)值(-7.3~-3.6)。大格勒农场南辉长岩的地球化学组成具有岛弧基性岩特征,其成因可能是流体和俯冲沉积物加入的富尖晶石橄榄岩部分熔融的结果。综合区域资料表明,大格勒农场南辉长岩形成于东昆仑原特提斯洋俯冲晚期阶段,进一步限定原特提斯洋晚期俯冲与碰撞转换时限为中志留世,随后转入碰撞造山阶段。

东昆仑东段原特提斯洋的俯冲时限:来自瓦勒尕花岗质岩石锆石U-Pb年代学及地球化学的约束

为探讨东昆仑东段早泥盆世原特提斯洋构造演化,本文对沟里地区瓦勒尕花岗质岩石进行了岩石学、锆石U-Pb年代学、地球化学等方面的研究。研究区花岗质岩石主要为二长花岗岩和花岗闪长岩,二长花岗岩锆石U-Pb年龄为414.2±2.3 Ma,花岗闪长岩的锆石U-Pb年龄为416.9±5.3 Ma。二长花岗岩和花岗闪长岩具富硅、富碱、富铝和弱过铝质特征,二者相对富集轻稀土元素,亏损重稀土元素,普遍具弱负铕异常,成因类型为I型花岗岩。研究认为,瓦勒尕二长花岗岩和花岗闪长岩为下地壳基性岩石部分熔融的产物,综合区域地质背景认为二者形成于原特提斯洋俯冲消减阶段,东昆仑地区原特提斯洋在414 Ma时尚未完全关闭。

青海沟里地区斜长角闪岩锆石U-Pb年代学、Lu-Hf同位素特征及其指示意义

东昆仑造山带金水口群的沉积和变质时代一直存在争议。本文利用LA-ICP-MS技术对青海省都兰县沟里地区金水口群中的斜长角闪岩进行了锆石U-Pb年代学、Lu-Hf同位素研究。结果显示:斜长角闪岩中6个测点的碎屑锆石年龄集中在2.1~2.0 Ga之间,碎屑锆石最小年龄为1829 Ma,变质锆石的上交点年龄为(1905±300)Ma,误差较大,但暗示金水口群沉积时间漫长,沉积时代不晚于中元古代早期,故本文认为金水口群沉积时代在1800 Ma左右;年龄集中于2100 Ma的碎屑锆石具有麻粒岩相变质锆石特征。变质锆石年龄集中于450~420 Ma之间,12个点的加权平均年龄为(444.3±5.3)Ma,为早古生代,该年龄与东昆仑造山带响应原特提斯洋俯冲拼合并发生区域深熔作用的年龄基本一致;锆石^(176)Lu/^(177)Hf值较低,变质锆石ε_(Hf)(t)值为-35.41~-3.75,T_(DM2)介于3641~1666 Ma之间,源岩物质为古老地壳再循环的产物;古元古代碎屑锆石ε_(Hf)(t)值为-2.98~25.58,T_(DM2)介于2833~920 Ma之间,源区为来源于亏损地幔的陆壳,与太古宙TTG岩系相似,暗示阿尔金地块的TTG岩系为金水口群提供了部分物质来源。金水口群沉积于活动大陆边缘或初始裂谷盆地,上部沉积盖层冰沟群与万宝沟群沉积于浅海陆棚,沉积过程具有连续性。

昆仑河早古生代两期埃达克质侵入岩的发现及其对东昆仑碰撞造山过程的启示

早古生代志留纪-泥盆纪是东昆仑原特提斯洋闭合和碰撞造山过程发生的重要时期。在该阶段出现有榴辉岩和大量A型花岗岩、镁铁-超镁铁质岩,并产出有夏日哈木镍矿、白干湖钨锡矿等一批重要的矿产资源。然而,东昆仑早古生代详细碰撞造山的深部过程、榴辉岩的折返机制等问题还没有得到合理的解释。在东昆仑造山带昆仑河地区新发现的早古生代埃达克质侵入岩可能为上述问题的解决提供了重要依据。LA-ICP-MS锆石U-Pb测年结果表明昆仑河地区存在晚奥陶世(446Ma)和晚志留世(427-425Ma)两期埃达克质侵入岩。晚奥陶世埃达克质侵入岩为花岗闪长岩,SiO2(67.55%-68.21%)和Al2O3(14.59%-15.89%)含量较高,富Na2O(4.91%-5.15%)、贫K2O(1.54%-1.64%),亏损重稀土,Y含量是7.76×10^(-6)-8.61×10^(-6),Yb含量是0.67×10^(-6)-0.93×10^(-6),高Sr(484×10^(-6)-621×10^(-6))和Sr/Y比值(56-80),Cr(31.2×10^(-6)-38.3×10^(-6))和Ni(19.0×10^(-6)-22.5×10^(-6))含量较高,具有亏损的全岩Nd同位素组成,εNd(t)为+1.1-+1.2,其岩浆成因锆石的εHf(t)分成两组:主要岩浆锆石为+2.2-+10.6,另外一组岩浆锆石为-11.5--1.3。晚志留世埃达克质侵入岩主要为花岗闪长岩、花岗岩,高SiO2(66.36%-71.14%)和Al2O3(15.5%-19.65%),富Na2O(5.65%-7.68%),贫K2O(0.68%-1.47%),强烈亏损重稀土,Y为1.90×10^(-6)-5.02×10^(-6),Yb为0.16×10^(-6)-0.51×10^(-6),高Sr(362×10^(-6)-1100×10^(-6))和Sr/Y比值(130-514),具有弱富集的全岩Nd同位素组成,εNd(t)为-1.5--1.2,岩浆锆石εHf(t)主要分布在-10.2-+9.3之间。本文认为晚奥陶世埃达克质侵入岩最可能是俯冲洋壳熔融形成的埃达克质岩浆在上升过程中与同期古老大陆地壳来源岩浆发生了混合后形成的,而晚志留世埃达克质侵入岩可能是加厚下地壳部分熔融形成。结合区域地质资料,本研究认为东昆仑在晚奥陶世(446Ma)还存在大洋板片的俯冲和熔融,约在436-425Ma发生了大陆碰撞、板片断离和高压-超高压(HP-UHP)变质岩折返,并诱发了埃达克质、镁铁-超镁铁质和A型花岗质岩浆的产生。

基于汇水盆地的化探异常识别与评价:以东昆仑乌拉斯太地区1∶5万水系沉积物地球化学测量为例

水系沉积物地球化学测量是卓有成效的找矿技术方法,基于汇水盆地的水系沉积物地球化学异常提取有助于追溯异常源头、指明找矿方向。基于SCB法提出了汇水盆地法,该方法以自然汇水盆地为预测单元,将各样品点的地球化学属性特征赋予各自所在区域,预测结果较为符合地形地貌特征,且对于采样密度较高的1∶5万水系沉积物地球化学测量数据有较好的应用效果。以东昆仑乌拉斯太地区为研究区,使用高精度DEM划分汇水盆地,利用该方法对1∶5万水系沉积物Au、Ag、Pb因子得分进行了异常信息提取,在提取过程中利用多种地貌参数(干流坡度、地形起伏比、汇水盆地面积)作为泥沙输移比进行残差校正计算,并在此基础上,利用C-A分形法分离了地球化学背景与异常。异常提取结果表明,汇水盆地面积参数最适合用作乌拉斯太地区的泥沙输移比参与顺流衰减校正,使用汇水盆地法可以有效识别和提取该地区地球化学异常信息。使用汇水盆地法提取的化探异常范围与矿床空间位置吻合度高,可以为进一步找矿工作提供有利的信息。

橄榄石成分对东昆仑夏日哈木超大型镍钴矿床岩浆过程的约束

橄榄石是与镁铁-超镁铁质岩浆有关的铜镍硫化物矿床中最主要的造岩矿物之一,其成分记录了丰富的镍成矿信息,并可反映母岩浆成分组成、结晶分异过程及后期物质交换等。东昆仑夏日哈木镍钴矿床是全球铜镍硫化物矿床勘查20余年来最为重要的找矿发现之一,将逐步成为中国重要的镍钴资源基地。本文在大量镜下岩相学观察的基础上,对夏日哈木矿床橄榄石进行系统电子探针分析。分析数据表明,夏日哈木橄榄石Fo值为83.01~90.12;Ni含量为676×10^(-6)~2845×10^(-6),Mn、Ca含量均较低,最大值分别为2457×10^(-6)和650×10^(-6)。“巨型”斜方辉石包裹的橄榄石成分自核心到边部Fo值和NiO含量均增高,表明含矿岩相形成时存在新鲜岩浆的补充,至少存在2次岩浆活动。橄榄石最高Fo值为90.12,反映夏日哈木镍钴矿床的母岩浆是Mg#值为0.73的高镁玄武质岩浆。硫饱和始于深部岩浆房母岩浆中少量橄榄石的结晶分异,早期熔离量较小,估算的橄榄石与硫化物熔体质量比大致为20∶1。本次研究利用橄榄石成分约束了夏日哈木矿床岩浆的多期次活动,限定了母岩浆的成分性质与硫化物的熔离时机,可为矿床成因研究提供借鉴。

东昆仑五龙沟金成矿系统:基本特征、成矿控制与勘查标志

五龙沟金矿田位于东昆仑造山带东段,金成矿与三叠纪岩浆作用、脆性和/或韧性变形显示密切时空联系。但是,金容矿地质体类型与控矿构造特征尚不清,金成矿受哪些关键要素控矿尚不明确。本文通过广泛深入的文献资料调研与详细的野外调查,详细阐明了五龙沟地区金矿床地质特征与成矿地质环境,系统梳理、总结与对比了不同地段金矿床的矿化类型、容矿地质体与控矿构造。研究表明,五龙沟金矿田发育微细脉型、蚀变岩型、石英脉型等3种矿化类型,容矿地质体主要以元古界—奥陶系变质岩系与侵入岩类为主。早期韧性剪切作用导致矿源层中金的活化与预富集,金矿化主要受控于晚期北西向脆性变形及相关断裂破碎带。大规模金成矿与古特提斯洋关闭及伴随的同/后碰撞构造过程有关。北西向构造—蚀变带、侵入岩类与古老变质基底是五龙沟金矿田的重要勘查标志。