Geochemistry of island arc assemblage in the Eastern Desert of Egypt and the role of Pan-African magmatism in crustal growth of the Arabian–Nubian Shield:A review

Neoproterozoic island arc assemblage of the Arabian–Nubian Shield(ANS)in the Eastern Desert(ED)of Egypt comprises juvenile suites of metavolcanics(MV),large amounts of meta-sedimentary rocks(MS),and voluminous metagabbros-diorites(MGD)and syn-tectonic intrusions of older granitoids(OG).We report here the updates of these four rock units in terms of classification,distribution,chemical characteristics,geodynamic evolution,metamorphism,and ages.In addition,we discuss these integrated data to elucidate a reasonable and reliable model for crustal evolution in the ANS.The main features of these rock units indicate their relation to each other and the geodynamic environment dominated by early immature oceanic island arcs to primitive continental arcs.Integrated information of the island arc metavolcanic and plutonic rocks(gabbros,diorites,tonalites,and granodiorites)furnish evidence of the genetic relationships.These include proximity and a coeval nature in the field;all protolith magmas are subalkaline in nature following calc-alkaline series with minor tholeiitic affinities;common geochemical signature of the arc rocks and subduction-related magmatism;their similar enrichment in LREEs;and similar major element compositions with mafic melts derived from metasomatized mantle wedge.The volcano-sedimentary and the OG rocks underwent multiphase deformation events whereas the MGD complexes deformed slightly.Based on the magmatic,sedimentological,and metamorphic evolutions constrained by geochronological data as well as the progressive evolutionary trend from extensional to compressional regimes,a possible gradual decrease in the subducted slab dip angle is the most infl uential in any geodynamic model for arc assemblage in the ED of Egypt.

Petrogenesis of Middle-Eocene granitoids and their Mafic microgranular enclaves in central Urmia-Dokhtar Magmatic Arc(Iran): Evidence for interaction between felsic and mafic magmas

Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0 ± 0.7 Ma for the granodiorite phase. Mafic microgranular enclaves(MMEs) are common in these plutons, and have relatively low SiO_2 contents(53.04-57.08 wt.%) and high Mg#(42.6-60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous(A/CNK = 0.69-1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO_2 contents(59.7-66.77 wt.%) and lower Mg#(38.6-52.2); they are considered a product of partial melting of the mafic lower crust. Chondritenormalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies(Eu/Eu* = 0.60-0.93). The host granodiorite samples yield(^(87)Sr/^(86)Sr);ratios ranging from 0.70498 to 0.70591,positive eNd(t) values varying from +0.21 to +2.3, and TDM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves(^(87)Sr/^(86)Sr)i = 0.705111-0.705113, ε_(Nd)(t)= +2.14 to +2.16, T_(DM2) = 697-785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantlederived mafic magmas in an active continental margin arc environment.

A PETROLOGICAL OVERVIEW OF THE KOHISTAN MAGMATIC ARC, NW HIMALAYA, N. PAKISTAN

The Kohistan Terrane in N. Pakistan occupies the suture zone between the collided Indian and Asian plates [1] . It formed primarily as an intra\|oceanic island\|arc crust between 125 and 90Ma prior to its accretion with the Asian plate along the Shyok suture. The final obduction of Kohistan onto the Indian plate in Early Eocene ensued the principal phase of the Himalayan orogeny.A two\|fold subdivision of the rocks forming the Kohistan crust is now well established [2] . Much of the ultramafic\|mafic component of the arc crust developed in the intraoceanic setting prior to collision with Asia. Mafic\|ultramafic plutonic complexes (Jijal and Sapat), together with gabbroic\|basaltic amphibolites (Kamila belt), at the base of the arc crust, and basaltic\|boninitic volcanics (the Chalt Formation) and Jaglot\|Yasin metasediments, in the upper arc crust are representative lithologies of this tectonic setting. Post Shyok suture lithologies include ca.80～45Ma Kohistan Batholith, Early Eocene Utror\|Shamaran volcanics and sedimentary rocks deposited in Early Eocene Dir and Drosh basins in Andean\|type setting. The Chilas gabbronorite\|ultramafic complex was magmatically emplaced in the waning stages of the intraoceanic phase but preceded the onset of the Andean\|type magmatism.

Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagze and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations, we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41 %-50.4% and 64 % -69 %, belonging to calc-alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content, 0.66%-1.01%, much lower than those of continental tholeiite. The ∑REE contents of basalt and dacite are 60.3-135 μg/g and 126, 4--167.9μg/ g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb = 0.54--1.17 averaging 0. 84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb= 0. 74 -1. 06 averaging 0. 86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone. The spinal Iherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.

Magmatism-Related Thermal Simulation of Volcanic Arcs in the Molucca Sea Bidirectional Subduction System

The bidirectional subduction system,island arc magmatic activities,and thermal structure of the forearc basin in the Molucca Sea are taken into consideration in this study.The active volcanic arcs on both sides of the bidirectional subduction zone in the Molucca Sea are undergoing arc-arc collisions.We applied a finite element thermal simulation method to reconstruct the thermal evolution history of the Molucca Sea Plate based on geophysical data.Then,we analyzed the thermodynamic characteristics of island arc volcanism on both sides of the bidirectional subduction zone.The results showed that at 10Myr,the oceanic ridge of the Molucca Sea Plate was asymmetrically biased to the west,causing this bidirectional subduction to be deeper in the west than in the east.Furthermore,the oceanic ridge subducted under the Sangihe arc at 5.5Myr,causing intermittent cessation of volcanic activities.Due to the convergence of bidirectional subduction,the geothermal gradient in the top 3km depth of the forearc area between the Sangihe and Halmahera arcs decreased from about 60℃km^(−1) at 4Myr to about 38℃km^(−1) today.Finally,within the 45–100 km depth range of the sliding surface of the subduction,anomalously high-temperature zones formed due to shear friction during the bidirectional subduction.

Constraints on early Paleozoic magmatic processes and tectonic setting of Inexpressible Island, Northern Victoria Land, Antarctica

During the Cambrian and Ordovician, widespread magmatic activity occurred in the Ross Orogen of central Antarctica, forming the Granite Harbor Intrusives and Terra Nova Intrusive Complex. In the Terra Nova Intrusive Complex, the latest magmatic activity comprised the emplacement of the Abbott Unit(508 Ma) and the Vegetation Unit(～475 Ma), which were formed in different tectonic settings. Owing to their similar lithological features, the tectonic transformation that occurred between the formation of these two units has not been well studied. Through a detailed geological field investigation and geochemical and geochronological analyses, four types of magmatic rock—basalt, syenite, mafic veins, and granite veins—were identified on Inexpressible Island, Northern Victoria Land. Our SHRIMP(Sensitive High Resolution Ion Micro Probe) zircon U–Pb ages of the basalt and the granite veins are 504.7 ± 3.1 and 495.5 ± 4.9 Ma, respectively. Major-and trace-element data indicate a continental-margin island-arc setting for the formation of these two rock types. The zircon U–Pb ages of the syenite and the monzodiorite veins are 485.8 ± 5.7 and 478.5 ± 4.0 Ma, respectively. Major-and trace-element compositions suggest a collisional setting for the former and an intracontinental extensional setting for the latter. These results elucidate the evolution from subduction to collision and intracontinental extension in Northern Victoria Land during the 20 Ma spanning the late Cambrian–Early Ordovician and improve our understanding of the tectonics and evolution of the Ross Orogen in the Transantarctic Mountains.

Geochemistry and Genesis of Iron-apatite Ore in the Khanlogh Deposit,Eastern Cenozoic Quchan-Sabzevar Magmatic Arc,NE Iran

The Khanlogh deposit in the Cenozoic Quchan-Sabzevar magmatic belt, NE Iran, is hosted by Oligocene granodioritic rock. The Khanlogh intrusive body is I-type granitoid of the calc-alkaline series. The orebodies are vein, veinlet, massive, and breccia in shape and occur along the fault zones and fractures within the host rock. Ore minerals dominantly comprise magnetite and apatite associated with epidote, clinopyroxene, calcite, quartz, and chlorite. Apatites of the Khanlogh deposit have a high concentration of REE, and show a strong LREE/HREE ratio with a pronounced negative Eu anomaly. Magnetites have a high concentration of REE and show weak to moderate LREE/HREE fractionation. They are comparable to the REE patterns in Kiruna-type iron ores and show an affinity to calc-alkaline magmas. The Khanlogh deposit is similar in the aspects of host rock lithology, alteration, mineralogy, and mineral chemistry to the Kiruna-type deposits. Field observations, hydrothermal alteration halos, style of mineralization, and the geochemical characteristics of apatite, magnetite, and host rock indicate that these magnetite veins have hydrothermal origin similar to Cenozoic Kiruna-type deposits within the Tarom subzone, NW Iran, and are not related to silica-iron oxide immiscibility, as are the major Precambrian magnetite deposits in central Iran.

Cretaceous magmatic arc in Hainan and the peri-South China Sea asevidenced by geochemical fingerprinting of granitoids in the region

Mesozoic magmatic rocks occur widely in the South China Block and are generally interpreted as the manifestations of the subduction of the Paleo-Pacific oceanic lithosphere beneath Asia.Subductiondriven magmatism in southeast(SE)China continued from the Late Permian through the Late Cretaceous with an inferred lull between 125 Ma and 115 Ma that is known in the literature as the Cretaceous"magmatic quiescence".We report in-situ zircon U-Pb ages,Hf-O and whole-rock Sr-Nd isotopes,and whole-rock geochemistry of Cretaceous granitoids on Hainan Island and discuss their magmatic evolution within the framework of the Late Mesozoic geodynamics of SE China.We recognize two main stages of the emplacement of Cretaceous granitoids on Hainan,first around 120 Ma and then around 100-95 Ma,displaying high-K calc-alkaline,I-type geochemical affinities.Granites in both age groups are enriched in LILE and LREE,but depleted in Nb,Ta,Ba,Sr,and Eu.The 120 Ma granites have zircon ε_(Hf)(t)values of-2.6 to 2.3 corresponding to Hf crustal model ages,ranging from 0.79 Ga to 1.03 Ga,and δ^(18)O values ranging from 6.9‰to 7.7‰.Zircons from 100-95 Ma granites have ε_(Hf)(t)values of-4.2 to 1.1 corresponding to Hf crustal model ages of 1.08 Ga to 1.42 Ga,and δ^(18)O values ranging from 6.7‰to 8.4‰.Increasing ε_(Hf)(t)values of the Cretaceous Hainan granites with younger crystallization ages indicate addition of more juvenile components and reworking of crustal material into their melt evolution.The ε_(Nd)(t)values of the 120 Ma and 100-95 Ma granitoids range between-4.1 to-0.4 and-7.7 to-4.0,respectively.The calculated two-stage model age of the 100-95 Ma granitoids clusters between 1.25 Ga and 1.53 Ga.These isotopic data suggest that magmas of the Cretaceous granitoids were produced by partial melting of Mesoproterozoic metabasaltic rocks,which make up much of the crystalline basement of the southern Cathaysia block.The geochemical and isotopic characteristics of the Cretaceous granitoids on Hainan resemble those of magmatic arcs in the Circum-Pacific orogenic belts and identical to those of nearly coeval granitoid intrusions in the continental fragments within the South China Sea basin.We interpret these Cretaceous granitoids in the Peri-South China Sea region as the remnants of a once contiguous Late Mesozoic magmatic arc system that bounded the southern margin of the entire continental Southeast Asia.Our findings do not support the existence of an episode of magmatic quiescence in the geological record of SE China during the Aptian.

Arc magmatism as a window to plate kinematics and subduction polarity:Example from the eastern Pontides belt,NE Turkey

The Eastern Pontides orogenic belt in the Black Sea region of Turkey offers a critical window to plate kinematics and subduction polarity during the closure of the Paleotethys. Here we provide a brief synthesis on recent information from this belt. We infer a southward subduction for the origin of the Eastern Pontides orogenic belt and its associated late Mesozoic--Cenozoic magmatism based on clear spatial and temporal variations in Late Cretaceous and Cenozoic arc magmatism, together with the exis- tence of a prominent south-dipping reverse fault system along the entire southern coast of the Black Sea. Our model is at variance with some recent proposals favoring a northward subduction polarity, and illus- trates the importance of arc magmatism in evaluating the geodynamic milieu associated with convergent margin orocesses.

Geochemistry of an Alaskan-type mafic-ultramafic complex in Eastern Desert, Egypt:New insights and constraints on the Neoproterozoic island arc magmatism

Mikbi intrusion(MI) is a part of the Neoproterozoic Nubian Shield located along the NE-SW trending major fracture zones prevailing southern Eastern Desert of Egypt. In this study, we present for the first time detailed mineralogical and bulk-rock geochemical data to infer some constraints on the parental magma genesis and to understand the tectonic processes contributed to MI formation. Lithologically, it is composed of fresh peridotite, clinopyroxenite, hornblendite, anorthosite, gabbronorite, pyroxene amphibole gabbro, amphibole gabbro and diorite. All rocks have low Th/La ratios(mostly <0.2) and lack positive Zr and Th anomalies excluding significant crustal contamination. They show very low concentrations of Nb, Ta, Zr and Hf together with sub-chondritic ratios of Nb/Ta(2-15) and Zr/Hf(19-35),suggesting that their mantle source was depleted by earlier melting extraction event. The oxygen fugacity(logfO_2) estimated from diorite biotite is around the nickel-nickel oxide buffer(NNO) indicating crystallization from a relatively oxidized magma. Amphiboles in the studied mafic-ultramafic rocks indicate relative oxygen fugacity(i.e. ΔNNO; nickel-nickel oxide) of 0.28-3 and were in equilibrium mostly with 3.77-8.24 wt.% H_2 O_(melt)(i.e. water content in the melt), consistent with the typical values of subduction-related magmas. Moreover, pressure estimates(0.53-6.79 kbar) indicate polybaric crystallization and suggest that the magma chamber(s) was located at relatively shallow crustal levels. The enrichment in LILE(e.g., Cs, Ba, K and Sr) and the depletion in HFSE(e.g., Th and Nb) relative to primitive mantle are consistent with island arc signature. The olivine, pyroxene and amphibole compositions also reflect arc affinity. These inferences suggest that their primary magma was derived from partial melting of a mantle source that formerly metasomatized in a subduction zone setting. Clinopyroxene and bulkrock data are consistent with orogenic tholeiitic affinity. Consequently, the mineral and bulk-rock chemistry strongly indicate crystallization from hydrous tholeiitic magma. Moreover, their trace element patterns are subparallel indicating that the various rock types possibly result from differentiation of the same primary magma. These petrological, mineralogical and geochemical characteristics show that the MI is a typical Alaskan-type complex.

Petrogenesis and Tectonic Significance of the Xiuwacu Two-Period Magmatism in Geza Arc of Yunnan Province： Constraints from Lithogeochemistry, Zircon U-Pb Geochronology and Hf isotopic Compositions

Objective The Late Cretaceous Xiuwacu ore-bearing porphyry is located in the Geza area of southern Yidun arc, SW China. In this area, the rock mass is mainly composed of three lithofacies： biotite granite porphyry, monzonitic granite and light alkali feldspar granite. As a part of the Yidun arc, the Geza arc has common structure and temporal- spatial evolution with the ~idun arc, which has experienced three stages of oceanic crust subduction, collision orogeny and intracontinent convergence stages. The molybdenite ores in the area are mainly hosted in monzonitic granite-porphyry and structural fracture zone, and the ore bodies are strictly controlled by faults. In recent years, great geological prospecting results have been achieved in Xiuwacu, and the deposit has reached a medium scale. However, there are few researches on the metallogenic porphyry. Based on the previous research, we determined the rock-forming and ore-forming age of the porphyry, and found that there were two stages of magmatism intrusion in Xiuwacu： Indosinian and Yanshanian. We also discussed the geochemical characteristics and source area of the rocks in the area.

The Late Cretaceous Crustal Magmatism of the Geza Arc Metallogenic Belt in Yunnan Province,and Zircon Ages and Hf Isotopic Evidence of the Porphyry Cu-Mo Mineralization

Objective The Geza arc in Yunnan Province,located in the southern Yidun arc,is an important part of the Sanjiang tectonic-magmatic belts in southwestern China and is a newly discovered copper polymetallic ore-concentrating district.Recent studies show that the newly discovered Yanshanian porphyry Cu-Mo polymetallic mineralization superimposed in the Indosinian porphyry copper belt in this area.

Crystallization Conditions and Mineral Chemistry in the East of Tafresh, Central Iran, with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA)in Iran.These rocks,mainly consisting of diorite and granodiorite,were emplaced during the Early Miocene.They are composed of varying proportions of plagioclase+K-feldspar+hornblende±quartz±biotite.Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field.The estimated pressure,derived from Al in amphibole barometry,is approximately 3 Kb.The granitoids are I-type,metaluminous and belong to the calc-alkaline series.They are all enriched in light rare earth elements and large ion lithophile elements,depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas.Most crystal size distribution(CSD)line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves,sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids.Moreover,the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years,which is indicative of shallow depth magma crystallization.

Control of subduction rate on Tonga-Kermadec arc magmatism

Dehydration/melting of oceanic crusts during returning to the mantle in subduction zones are related to origin of arc lavas. The factors that influence arc magmatism include compositions of the subducting slabs, mantle wedge and subduction rates. However, distinguishing these factors remains difficult and highly debated. Subducting rate is related to the total mass of inputs and controls thermal structure, thus plays a crucial role in arc magmatism. Here we explore the relationships between geochemical variations of arc lavas and convergence rates(increasing from 46 mm/a to the south to 83 mm/a to the northward) in the Tonga-Kermadec arc system. Data of geochemistry for lava samples from nine islands of this arc system are collected and compiled to investigate the role of subduction rate in arc magmatism. Lavas from the northern Tonga arc with a faster subduction rate show broadly lower concentrations of TiO_2 and highfield-strength elements(HFSEs, e.g. Nb, Ta, Zr, Hf), and higher Ba/Th, U/Th ratios than the Kermadec Arc to the south. Some of the Kermadec lavas show the highest values of Th/Nb ratio. We suggest that the northern Tonga arc with a higher subduction rate has been influenced by a stronger role of subductionreleased fluid, which results in stronger large-ion-lithophile elements(LILEs) and relatively weaker HFSEs contribution. It is interpreted that faster subduction rate tend to create a cooler subduction zone, leading to stronger dehydration subduction slab contribution with, thus, higher LILE/HFSE ratios of arc lavas. The conclusion contributes to a better understanding of arc magmatism, and ultimately the long-term chemical differentiation of the Earth. More supplementary geochemical data along Tonga-Kermadec arc and tests in other arcs are needed.

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex,North Qaidam:Contraints on the Nature of the Alaskan-type Continental Arc Root

Orogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity,slab subduction,and melt or fluid metasomatism.To determine the source of the mantle-derived parental magma of the peridotite and to investigate the metasomatism that it experienced,we undertook an integrated study of the petrography,whole-rock major-and trace-element compositions,in situ zircon U-Pb geochronology,and mineral majorand trace-element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam.The Halihatu ultramafic-mafic complex is composed of dunite,pyroxene peridotite,and gabbro,which are characteristic of Alaskan-type complexes.The dunite yields a weighted mean^(206)Pb/^(238)U age of 479±5 Ma(MSWD=0.7),which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma.The peridotites have high Mg^(#)(89.8-91.8)and Cr contents(2419-5190 ppm),low Al_(2)O_(3)(0.20-1.68 wt%)and Ni(289-1012 ppm)contents,and high olivine Fo contents(87-91),suggesting a large degree(~15%-22%)of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene.This is consistent with estimates of 15%-22.3%partial melting calculated using the Cr#of spinel crystals and with the low Yb(0.04-0.33 ppm)and Y(0.72-1.29 ppm)contents of clinopyroxene crystals.Whole-rock trace-element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements,along with high Al_(2)O_(3)(2.10-6.47 wt%)and low TiO_(2)(0.01-0.21 wt%)contents of clinopyroxene crystals,suggesting an arc magma cumulate trend.These features,along with the high olivine Fo contents(87-91 ppm),imply that the Halihatu peridotite is an Alaskan-type crustal cumulates derived from Mgrich hydrous basaltic melts.The high estimated f O_(2)(FMQ+1.97 to FMQ+3.81)further supports the idea that they formed in an arc setting.The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine,quenched boundaries between mafic and felsic melts,and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust.Therefore,we propose a new cumulate-infiltration model for the petrogenesis of Alaskan-type ultramafic complexes,which improves our understanding of the nature of Alaskan-type continental arc root.

云南墨江金矿区内凝灰岩的成因及对金成矿作用的意义

云南墨江金厂金矿是哀牢山金矿带内的一个大型金矿床,矿床受剪切带控制,矿体赋存于古生代浅变质岩中,其成矿物质来源一直是争论的焦点。为进一步精确约束赋矿地层的形成时代和大地构造背景,探讨其与金成矿物质来源和成矿作用之间的关系,本文对古生代浅变质岩中的凝灰岩开展了年代学、地球化学、黄铁矿成分和同位素特征研究。LA-ICP-MS锆石U-Pb定年结果(256.6±2.3Ma)表明墨江金矿区内存在晚二叠世火山碎屑岩。锆石稀土元素含量较高,Ce正异常(δCe=2.02~32.98)和Eu负异常明显(δEu=0.06~0.38)。岩石具有低钛(TiO_(2)=0.47%~0.71%)、较高铝(Al_(2)O_(3)=10.96%~16.33%)、富铁(Fe_(2)O_(3)T=3.21%~9.12%)和硫(S=0.95%~3.92%)等特征,属于英安质-流纹质凝灰岩;此外,所有样品轻重稀土元素分馏显著((La/Yb)_(N)=7.50~15.7),Eu异常明显(δEu=0.54~0.68),亏损Nb、Ta、Ti、Sr、Ba,富集K、Pb,显示出弧火山岩的地球化学特征。Nd同位素(ε_(Nd)=-9.24~-9.61)显示富集特征,二阶段钕模式年龄为1.61~1.79Ga。凝灰岩中富含黄铁矿,黄铁矿属铁亏损型(S/Fe=2.00~2.09),存在Ni对Fe、As对S的类质同象置换;S主要来自地壳(δ^(34)SV-CDT=-8.8‰~-9.7‰);Co、Ni含量高但没有相关性,指示其为火山成因,后期受到了富Ni流体交代;黄铁矿的Au含量偏高,但未发现独立金矿物,推测可能存在纳米金。全岩地球化学特征、Nd同位素特征和锆石微量元素特征指示墨江金矿区凝灰岩形成于陆缘弧构造背景,为古特提斯洋俯冲消减过程中玄武质岩浆底侵导致古元古代变质基底重熔的产物,岩浆形成过程中伴有地幔物质的加入,同时存在金的初始富集,形成含金黄铁矿,并在随后的区域变质作用及构造热事件中发生金的活化迁移,为金矿床的最终形成提供成矿物质。晚二叠世弧岩浆作用对于墨江金矿的形成具有重要意义。

甘肃合黎山古元古代花岗岩锆石U-Pb年龄、岩石地球化学特征及其对阿拉善地块南缘构造环境的约束

分布于阿拉善地块南缘的龙首山岩群是金川镍矿的直接围岩,其形成时代和构造背景存在较大争议。在阿拉善地块南缘新发现的侵入于龙首山岩群的合黎山古元古代花岗岩体岩性为二长花岗岩、正长花岗岩及正长岩。岩石地球化学分析结果显示,合黎山花岗岩Al_(2)O_(3)/(CaO+Na_(2)O+K_(2)O)值为0.91~1.06,K_(2)O/Na_(2)O为1.10~1.44,属钾质碱钙性花岗岩,岩石相对富集Rb、Ba、Th、K等大离子亲石元素,亏损Ta、Nb、P、Zr、Hf、Ti等高场强元素,具有弧岩浆的地球化学特征,形成于俯冲带岩浆弧构造环境。LA−ICP−MS锆石U−Pb测年结果表明,合黎山二长花岗岩的形成年龄为1859±12 Ma,侵位于古元古代晚期,揭示龙首山岩群变质地层的形成时代大于1.85 Ga,反映阿拉善地块南缘在古元古代晚期可能处于俯冲消减过程。

辽东兰花岭地区古元古代Ⅰ型花岗岩类与辽吉A型花岗岩对比研究

辽东半岛是华北克拉通胶-辽-吉古元古代活动带的重要组成部分,古元古代经历了复杂的构造演化过程,并记录了多期岩浆-变质作用,约2.2 Ga的辽吉A型花岗岩和1.89~1.85 Ga的巨斑状花岗岩、正长岩分别标志着辽东古元古代造山作用的开端和结束。最新研究显示,2.20~2.15 Ga的岩浆作用形成了2种不同类型的花岗岩,它们可能具有不同的岩石成因和构造意义。在青城子铅锌矿集区北部采集的兰花岭、白砬子花岗闪长岩和黄泊辉绿岩,锆石U-Pb年龄分别为2177±19 Ma、2129±36 Ma、1876±29 Ma。花岗闪长岩的岩石成因类型、地球化学特征与典型的约2.2 Ga的辽吉A型花岗岩明显不同,属于弱过铝质、低钾钙碱性—碱性岩石,Zr、Hf、Nb、Rb含量较低,K_(2)O/Na_(2)O值、稀土元素总量极低,为典型的Ⅰ型花岗岩类。根据锆石Lu-Hf同位素分析,ε_(Hf)(t)值为-5.1~9.0,二阶段Hf模式年龄t_(DM2)为2089~2817 Ma,岩浆源区为约2.5 Ga的太古宙地壳物质和少量软流圈地幔物质。兰花岭地区花岗闪长岩具备岛弧或活动大陆边缘的地球化学亲缘属性,可能形成于弧岩浆俯冲挤压环境;结合形成于伸展环境的A型条痕状花岗岩特征,认为约2.2 Ga辽东地区古元古代活动带呈现总体伸展、局部挤压的构造环境,为洋壳板块向龙岗地块俯冲碰撞过程中或碰撞后的弧后盆地。

滇东地区裂谷盆地内火山-侵入岩序列与金属成矿

碱性岩浆对金属成矿和金属矿集区有独特控制作用,因火山—侵入岩序列与金属成矿和岩浆叠加侵入构造系统识别难度大,长期制约了深部勘探和找矿预测。本文以云南东川铁铜金矿集区(元古宙陆缘裂谷盆地)和个旧锡铜钨铯铷多金属矿集区(三叠纪弧后裂谷盆地)为主,采用大比例尺构造岩相学填图新方法,解析研究了碱性岩浆活动有关的火山—侵入岩侵入序列与岩石组合、裂谷盆地演化和岩浆叠加侵入构造系统,揭示了它们与滇东地区铁氧化物铜金型(IOCG)、铜钴-金红石-稀土和锡铜钨铯铷多金属矿叠加成矿内在关系。研究认为:①云南东川铁铜金矿集区内,新太古代末期碱性岩类以方解石钠长岩(2520±14 Ma)和碱性铁质苦橄岩(2529±77 Ma)为主。新太古界—古元古界小溜口岩组(>2.50Ga)顶部古岩溶风化壳,受中元古代火山喷发—岩浆侵入构造叠加再造形成了特殊单元(独立填图单元,2.50~1.80Ga),它们是铜钴-金红石-稀土矿床的新找矿层位。②在中元古界因民组一段内,铁钠质碱性基性岩、铁钾质粗面岩和铁质碱性辉绿辉长岩等岩石组合形成于因民期火山断陷成盆期。因民组二段和三段是稀矿山式铁铜矿床储矿层位,铁铜矿层下盘铁钾质粗面岩为独居石型稀土成矿层位。在格林威尔造山期碱性钛铁质辉长岩(1097~1047 Ma)-碱性钛铁质闪长岩-碱性二长斑岩等碱性钛铁质侵入岩体,形成了白锡腊深部IOCG矿床和金红石富集成矿。③在个旧三叠纪弧后裂谷盆地内,三叠纪碱性苦橄岩和碱玄岩中形成了Sn-Cu-Zn-Li-Rb-Cs初始富集;在晚白垩世浅色花岗岩叠加成矿作用下,形成了金云母矽卡岩型锡铜钨铯铷矿床。④裂谷盆地内早期碱性岩中初始富集金属与后期富含成矿金属的碱性侵入岩叠加成矿可能是金属超常富集机理。在东川地区岩浆叠加侵入构造内,岩浆热液角砾岩筒对铜钴-金红石-稀土元素叠加富集成矿控制显著;在个旧地区,浅色花岗岩侵入于碱性苦橄岩-碱玄岩中,对金云母矽卡岩型锡铜钨钴铯铷矿床的叠加富集成矿控制显著。

活动大陆边缘岩浆作用及构造演化——以敦煌地块为例

大陆边缘弧是汇聚板块边界与俯冲有关的岩浆作用产物,通常记录了造山带弧岩浆作用、造山过程和大陆地壳形成与演化等诸多重要的地质过程。作为中亚造山带中段最南部的构造单元,敦煌地块为传统定义上具有早前寒武纪变质结晶基底的微陆块,其后在古生代时期经历了多期次、多阶段的与中亚造山带造山相关的构造热事件并使其发生再活化,进而产生了一系列广泛的弧岩浆-变质作用事件。然而,对于该陆块古生代弧岩浆作用机制和地壳构造演化历史缺乏系统的研究。本文综合近十多年来对敦煌地块的诸多最新研究进展,系统梳理了古生代岩浆岩岩石组合类型、年代格架、地球化学组成以及同时期变质-沉积构造热事件演化特征,得到以下认识:(1)敦煌地块古生代岩浆作用过程呈现阶段性特征,即幕式岩浆作用,构造位置上从东北部逐渐迁移到南部再折返到中部,大致可划分为五期:~510Ma、440~410Ma、390~360Ma、~330Ma和280~245Ma,而变质作用事件主要集中在450~360Ma;(2)古生代岩浆岩类型主要以钙碱性I型花岗岩、埃达克质岩石、少量S型花岗岩和高钾花岗岩为主,且岩石成分从寒武纪低钾拉斑系列逐渐转变为二叠纪高钾、富硅特征;(3)同位素地球化学特征表明,敦煌地块中-北部寒武纪-泥盆纪发育的与俯冲相关的弧岩浆对新生地壳的生长起了重要贡献,并伴随古老地壳再造事件;而南部泥盆纪-石炭纪岩浆作用则以古老地壳物质重熔为主;(4)基于埃达克质岩石的证据,敦煌地块在古生代时期经历了两次显著的地壳增厚事件,早期可能是与北山南部石板山地体/弧碰撞以及幔源岩浆底侵有关,而晚期可能是与俯冲板片后撤或回卷相关,地壳厚度可达~55km;(5)敦煌地块属于中亚造山带中段最南部一个具有前寒武纪基底的古老微陆块,后期卷入了古亚洲洋南向俯冲相关的造山事件使其被强烈破坏与改造。该微陆块作为古亚洲洋南部的活动大陆边缘弧,被与俯冲作用有关的阶段性弧岩浆底侵、部分熔融、增厚地壳和区域性变质作用等机制改造与活化,产生了阶段性侵位的陆缘弧岩浆作用。这些认识为探究中亚造山带微陆块的起源和造山带的构造演化提供新的约束。