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1.
《International Geology Review》2012,54(15):1776-1800
The northern and southern zones of the eastern Pontides (northeast Turkey) contain numerous plutons of varying ages and compositions. Geochemical and isotopic results on two Hercynian granitoid bodies located in the northern zone of the eastern Pontides allow a proper reconstruction of their origin for the first time. The intrusive rocks comprise four distinct bodies, two of which we investigated in detail. Based on LA–ICP–MS U–Pb zircon dating, the Derinoba and Kayadibi granites have similar 206Pb/238U versus 207Pb/235U Concordia ages of 311.1 ± 2.0 and 317.2 ± 3.5 million years for the former and 303.8 ± 1.5 million years for the latter. Aluminium saturation index values of both granites are between 0.95 and 1.35, indicating dominant peraluminous melt compositions. Both intrusions have high SiO2 (74–77 wt.%) contents and show high-K calc-alkaline and I- to S-type characteristics. Primitive mantle-normalized element diagrams display enrichment in K, Rb, Th, and U, and depletion in Ba, Nb, Ta, Sr, P, and Ti. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies with Lacn/Ybcn?=?4.6–9.7 and Eucn/Eu*?=?0.11–0.59 (Derinoba), and Lacn/Ybcn?=?2.7–5.5 and Eucn/Eu*?=?0.31–0.37 (Kayadibi). These features imply crystal-melt fractionation of plagioclase and K-feldspar without significant involvement of garnet. The Derinoba samples have initial ?Nd values between –6.1 and –7.1 with Nd model ages and T DM between 1.56 and 2.15 thousand million years. The Kayadibi samples show higher initial ?Nd(I) values, –4.5 to –6.2, with Nd model ages between 1.50 and 1.72 thousand million years. This study demonstrates that the Sr isotope ratios generally display negative correlation with Nd isotopes; Sr isotope ratios were lowered in some samples by hydrothermal interaction or alteration. Isotopic and petrological data suggest that both granites were produced by the partial melting of early Palaeozoic lower crustal rocks, with minor contribution from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the eastern Pontides.  相似文献   

2.
Mineral chemistry, major and trace elements, 40Ar/39Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. 40Ar/39Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (LaN/LuN = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial 87Sr/86Sr values vary between .70615 and .70796, whereas initial 143Nd/144Nd values change between .51228 and .51249. Initial 206Pb/204Pb values vary between 18.001 and 18.349, 207Pb/204Pb values between 15.611 and 15.629 and 208Pb/204Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.  相似文献   

3.
The geochemical and Sr–Nd–Pb isotope properties, as well as the Laser Ablation Inductively Coupled Plasma and Mass Spectrometry (LA-ICP-MS) U–Pb zircon age, of E?rikar Monzogranite in the eastern Pontides, are primarily investigated in this study with the aim of determining its magma source and geodynamic evolution. The U–Pb zircon age obtained from E?rikar Monzogranite is 78 ± 1.5 Ma, thereby re?ecting the age of monzogranite. The I-type E?rikar Monzogranite comprises quartz, plagioclase (An35–45), orthoclase, muscovite, and biotite. The geochemical analyses of the E?rikar Monzogranite indicate being medium K calc-alkaline, peraluminous, and resembling magmatic arc granite. The E?rikar Monzogranite is enriched in large ion lithophile elements and light rare earth elements relative to high field strength elements. Chondrite-normalized rare earth element patterns have concave upward shapes (LaN/YbN 2.47–8.58) with pronounced negative Eu anomalies (EuN/Eu* = 0.29–0.65). Initial εNd(i) values vary between 1.85 and 2.18 and initial 87Sr/86Sr values between 0.7048 and 0.7067. Fractionation of plagioclase, hornblende, and apatite played an important role in the evolution of E?rikar Monzogranite. The crystallization temperatures of the melts ranged from 770°C to 919°C based on zircon and apatite saturation temperatures. The geochemical and isotopic data suggest being generated by the partial melting of ma?c lower crustal sources.  相似文献   

4.
The Dahongliutan granitic pluton consists of two-mica granites and is located in the eastern part of the Western Kunlun Orogen, northwestern Tibetan Plateau. Zircon separates from the pluton yield a SIMS U–Pb age of 217.5 ± 2.8 Ma. Rocks from the pluton contain relatively high and uniform SiO2 (72.32–73.48 wt%) and total alkalis (Na2O + K2O = 8.07–8.67 wt%) and are peraluminous and high-K calc-alkaline to shoshonitic in composition. The Dahongliutan granites are relatively depleted in the high-field-strength elements and the heavy rare earth elements (HREEs) and have relatively high Rb, and low Ba and Sr concentrations. They contain low total rare earth element (REE) concentrations. The light REEs are strongly enriched relative to the HREEs, with (La/Yb)N values of 28.56–37.01. The εNd(t) values range from ?10.6 to ?8.8, and (87Sr/86Sr)i = 0.7142–0.7210. Zircons from the pluton yield εHf(t) values of ?13.8 to ?1.6, and δ18O = 10.5–11.6‰. Petrographic and geochemical features of the pluton indicate that the granites are S-type and were derived from parting melting of a mixture of metasedimentary and minor metaigneous sources in the middle–lower crust. Magmatic differentiation was dominated by the fractional crystallization of plagioclase, K-feldspar, muscovite, biotite, and accessory monazite, allanite, and Fe–Ti oxides. Regional granitoids were emplaced in the Early-to-Middle Triassic. Other younger granitoids, with ages of 240–200 Ma, are mostly I-type in character and were likely derived from multiple types of source rock, suggesting the source was heterogeneous Triassic crust. Such a scenario is consistent with their formation in a post-collisional setting. Our new data, combined with other geological evidence, suggest that the collision between the Tianshuihai and southern Kunlun terranes occurred between ca. 250 and 240 Ma, resulting in the closure of the Palaeo-Tethys. Post-collisional tectono-magmatic events may have occurred between 240 and 200 Ma.  相似文献   

5.
《China Geology》2022,5(4):579-594
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6.
Calc-alkaline arc magmatism at convergent plate margins is volumetrically dominated by metaluminous andesites. Many studies highlighted the importance of differentiation via fractionation processes of arc magmas, but only in the last decades, it has been demonstrated that not all rock-forming minerals may affect the evolution of calc-alkaline suites. In particular, a major role exerted by Al-rich hornblende amphibole as fractionating mineral phase has been documented in many volcanic arc settings. The aim of this work is to understand the role of the Tschermak molecule (CaAlAlSiO6) hosted in the hornblende and plagioclase fractionation assemblage in driving magma differentiation in calc-alkaline magmatic suites. We explore this issue by applying replenishment–fractional crystallization (RFC) and rare earth element–Rayleigh fractional crystallization (REE-FC) modeling to the Sabzevar Eocene (ca. 45–47 Ma) calc-alkaline volcanism of NE Central Iran, where hornblende-controlled fractionation has been demonstrated. Major element mass balance modeling indicates RFC dominated by a fractionating assemblage made of Hbl52.0–52.5 + Pl44.1–44.2 + Ttn3.3–3.9 (phases are expressed on total crystallized assemblage). REE-FC modeling shows, instead, a lower degree of fractionation with respect to RFC models that is interpreted as due to hornblende and plagioclase resorption by the residual melt. Calculations demonstrate that fractionation of the Tschermak molecule can readily produce dacite and rhyolite magmas starting from a calc-alkaline andesite source (FC = ca. 30 %). In particular, the Tschermak molecule controls both the heavy rare earth elements (HREE) and light rare earth element (LREE) budgets in calc-alkaline differentiation trends.  相似文献   

7.
《International Geology Review》2012,54(10):1113-1144
The Kumbalgarh Group of the south Delhi fold belt are the main bedrock series exposed in the axial region of the Aravalli craton. Quartzites and greywackes, the chief clastic constituents of this group, are well exposed. Petrographic and bulk-rock analyses of these rocks permit determination of their provenance, tectonic setting of the basin, and the Archaean to Proterozoic crustal evolution. Greywackes comprise quartz, plagioclase, amphiboles, K-feldspar, and rock fragments. Based on mineralogy, we divided the quartzites into three categories: QTZ1 is chiefly composed of quartz with a silty matrix and a minor quantity of feldspars and QTZ2 contains significant mafic minerals as well as quartz and feldspars, whereas QTZ3 is more feldspathic than the other groups. All the lithounits have SiO2/Al2O3 ratios <～10 suggesting textural immaturity consistent with their sedimentary petrography. Greywackes display the least fractionated rare earth elements (REEs) (La/Yb N : avg. 2.55) with positive Eu anomalies (avg. Eu/Eu* = 1.34). QTZ1 contains strongly fractionated REE patterns (avg. La/Yb N : 13.56, avg. Eu/Eu* = 0.60), QTZ2 shows moderate REE fractionation (avg. La/Yb N : 4.97, avg. Eu/Eu* = 0.61), and QTZ3 possesses the least fractionated V-shaped REE patterns (avg. La/Yb N : 1.97, avg. Eu/Eu* = 0.51). Weathering attributes including chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and A–CN–K plots assign a low to moderate degree of weathering to the Kumbalgarh sediments under a subtropical climate. Based on our synthesis of the petrographic and geochemical data, we suggest a provenance comprising basalts, tonalite–trondhjemite–granodiorite (TTG), and granite. Geochemical attributes indicate deposition of the detritus in an extensional backarc basin receiving sedimentary input from opposite directions. The opening and then closure of the South Delhi Basin was the last phase of the break-up of the supercontinent, columbia, which began by abortive rifting of the Udaipur belt and culminated in separation of the Aravalli–Bundelkhand–Dharwar block in the east and the East African orogen in the west.  相似文献   

8.
The modal and chemical composition of sands from Cox’s Bazar beach (CBB) and Kuakata beach (KB) areas of Bangladesh has been investigated to infer their maturity, chemical weathering, and provenance signatures. The CBB and KB sands are typically high quartz, low feldspar, and lithic fragments, representing a recycled orogen source. Major element compositions of CBB sands are characterized by high SiO2 (83.52–89.84 wt%) and low Al2O3 (4.39–6.39 wt%), whereas KB sands contained relatively low SiO2 (63.28–79.14 wt%) and high Al2O3 (9.00–11.33 wt%) contents. The major, trace and rare earth element (REE) compositions of beach sands display comparable distribution patterns with enriched Th and SiO2 for both sands relative to upper continental crust (UCC). Pb, Rb, Y, and Fe for KB sands are little higher than UCC and the rest of the elements are marked depleted for both suites reflecting destruction of plagioclase and K-feldspar during fluvial transportation. The CBB and KB sands are compositionally low mature to immature in nature subsequently classified as subarkose and litharenite, respectively. Chondrite-normalized REE patterns for CBB and KB sands show LREE enrichment and nearly flat HREE (LaN/YbN, 7.64–9.38 and 5.48–8.82, respectively) coupled with prominent Eu anomalies (Eu/Eu*, 0.51–0.72 and 0.52–0.76, respectively), suggesting felsic source provenance. The provenance discrimination diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE (∑LREE/HREE, Eu/Eu* and GdN/YbN) parameters indicate that CBB and KB sands were largely derived from felsic source rocks, with compositions close to average rhyolite, granodiorite, granite, and UCC.  相似文献   

9.
Geological mapping and zircon U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating has identified a Mesoarchean (2857 ± 17 Ma) geological unit in the Luanjiajie area of the northern margin of the North China Craton, within the northern part of Liaoning Province, China. This unit is dominated by tonalitic and trondhjemite gneisses that form part of a typical tonalite–trondhjemite–granodiorite (TTG) rock assemblage. These Mesoarchean gneisses are enriched in Na and depleted in K, yield K2O/Na2O ratios of 0.34–0.50, have Rittmann index (σ) values of 1.54–3.04, and are calc-alkaline. They have EuN/EuN* values of 0.77–1.20 (average of 1.03), indicating that these samples have negligible Eu anomalies, and yield high LaN/YbN values (4.92–23.12). These characteristics indicate that these Mesoarchean gneisses have fractionated rare earth element (REE) compositions that are enriched in the light REE (LREE) and depleted in the heavy REE (HREE), with steeply dipping chondrite-normalized REE patterns. These gneisses are also enriched in Rb, Th, K, Zr, and Hf, and are relatively depleted in Ta, Nb, P, and Ti. In summary, the magma that formed these tonalitic and trondhjemite gneisses was most likely derived from the partial melting of lower-crustal basaltic rocks during subduction. The timing of formation (2.85 Ga) of the Luanjiajie tonalite and trondhjemite gneisses probably represents the timing of initiation of plate tectonics within the LongGang Block during a SE-directed subduction event. The presence of inherited zircons with ages of >3.0 Ga within the Luanjiajie gneisses suggests that this area may contain as yet undiscovered rocks that formed before 3.0 Ga.  相似文献   

10.
The geological record of the Neoproterozoic to early Palaeozoic Proto-Tethyan Ocean in Southeast Asia is not clear. To better constrain the evolution of the Proto-Tethys, we present new geochronology, geochemistry, and petrology of the late Cambrian to Ordovician Pinghe pluton monzogranite from the Baoshan block, western Yunnan, southwest China. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of four zircon samples yield ages of 482–494 and 439–445 Ma for the pluton, interpreted as two episodes within one magmatic event accompanying the whole process of subduction–collision–orogeny between buoyant blocks and oceanic crust of the Proto-Tethys. The monzogranite belongs to the strong peraluminous, high-K, calc-alkaline series and shows characteristics of both I-type and S-type granitic rocks. It is characterized by extremely high Rb/Sr and Rb/Ba but low TiO2, MgO, FeOt, and CaO/Na2O ratios. The monzogranite is also moderately enriched in light rare earth elements (LREEs), depleted in heavy rare earth elements (HREEs), lacks HREE fractionation, and has strongly negative Eu (Eu/Eu* = 0.06–0.49), Ba, Nb, Ta, Sr, and Ti anomalies. Whole-rock εNd(t) and εHf(t) values range from ?8.7 to ?11.6 and ?5.55 to ?9.58, respectively. Nd and Hf two-stage model ages range from 1.66 to 2.06 Ga and 2.14 to 3.00 Ga, respectively, with variable radiogenic 206Pb/204Pb(t) (16.547–18.705), 207Pb/204Pb(t) (15.645–15.765), and 208Pb/204Pb(t) (38.273–38.830). These signatures suggest that the monzogranite magma was derived from partial melting of heterogeneous metapelite, which was generated from Neoarchean to Palaeoproterozoic materials mixed with basaltic magma. The monzogranite magma underwent crystallization differentiation of plagioclase, K-feldspar, and ilmenite. Magmatism to form the Pinghe pluton occurred in a post-collisional setting. Based on the comparison of coeval granites throughout adjacent regions (e.g. Himalayan orogen, Lhasa Terrane, and parts of Gondwana supercontinent), we propose that the Baoshan block was derived from the northern Australian Proto-Tethyan Andean-type active continental margin of Gondwana and experienced subduction of the Proto-Tethyan oceanic crust and accretion of an outboard micro-continent. The Pinghe pluton could have formed when a subducting oceanic slab broke off during collision.  相似文献   

11.
Chemical composition of rock-forming minerals in Appalachian Siluro-Devonian granitoid intrusions, southwestern New Brunswick, was systematically determined by electron microprobe. The mineral chemical data together with petrographic examination was used to test magmatic equilibration and to constrain crystallization conditions, volatile exsolution, and fluorine-chlorine activity of fluids associated with these intrusions. Mineralogical distinction between Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS) and Late Devonian granitic series (GS) rocks is evident, although both are subsolvus I-type to evolved I-type granitoids. Oxidized to reduced GMS rocks consist of quartz, plagioclase (An>10), K-feldspar, biotite, apatite, titanite, zircon, monazite, ± hornblende, ± pyroxene, ± magnetite, ± ilmenite, and ± sulfide. GS rocks comprise quartz, K-feldspar, plagioclase (An<10), mica group minerals, zircon, monazite, apatite, sulfide, ± ilmente, ± magnetite, ± topaz, ± columbite, and ± xenotime. Inter-intrusion and intra-intrusion variations in mineral chemistry are interpreted to reflect petrogenetic processes (e.g., assimilation and fractional crystallization) during granitoid evolution. Although magmatic equilibration among rock-forming minerals are disturbed by subsolidus hydrothermal processes, GMS rocks appear to have higher magmatic temperatures, variable levels of emplacement, a range of (i.e., reduced intrusions 10−16.7∼10−13.4 and oxidized intrusions 10−14.0∼10−10.5 bars), and relatively low f HF/f HCl ratios (10−3.0∼10−1.0) in exsolved fluids, compared to GS rocks. Reduced GMS intrusions bear higher gold potential and thus may be prospective targets for intrusion-related gold systems. Electronic Supplementary Material Supplementary material is available for this article at  相似文献   

12.
The southernmost Guyana Shield-Uatumã subdomain, northeastern Amazonas State, Brazil is dominantly formed by granitoid and volcanic rocks from the Água Branca Suite (ABS), undivided Granite Stocks (GS) and São Gabriel volcano–plutonic system (SGS). The ABS is characterized by a granite series that exhibits comparatively low Fe/(Fe + Mg) ratio, low (Nb/Zr)N, high Sr values and high Rb/Zr ratio. Its rocks display metaluminous to weakly peraluminous (A/CNK 0.94–1.06), high-K calc-alkaline, I normal-type character and have moderately to strongly fractionated rare earth elements (REE) pattern. The SG granites and SGS effusive–ignimbrite–granite association is metaluminous to weakly peraluminous (A/CNK 0.84–1.18), high-K calc-alkaline, has moderately to weakly fractionated REE trend, higher Fe/(Fe + Mg) ratio, lower Sr content and lower Rb/Zr ratio. The ABS geochemical signature is consistent with formation from volcanic arc rocks and small participation of collisional setting rocks, whereas the SG and SGS have post-collisional tectonic rocks-related geochemical signature. This model is in harmony with a post-collisional extensional regime, started with the 1.90–1.89 Ga Água Branca magmatism, and culminated with the 1.89–1.88 Ga São Gabriel system at an early stage of intracratonic reactivation, which included intrusion of mafic dikes. The Uatumã subdomain was related to mantle underplating with continental uplift and its origin involved contributions of 2.3–2.44 Ga Archean-contaminated Trans-Amazonian, 2.13–2.21 Ga Trans-Amazonian, 1.93–1.94/2.0 Ga Tapajós-Parima. Foliation styles point out that part of the Água Branca granitoids recorded later deformational effects, likely related to the Rio Negro Province formation.  相似文献   

13.
Amphibole-bearing, Late Archean (2.73–2.68 Ga) granitoids of the southern Superior Province are examined to constrain processes of crustal development. The investigated plutons, which range from tonalite and diorite to monzodiorite, monzonite, and syenite, share textural, mineralogical and geochemical attributes suggesting a common origin as juvenile magmas. Despite variation in modal mineralogy, the plutons are geochemically characterized by normative quartz, high Al2O3 (> 15 wt%), Na-rich fractionation trends (mol Na2O/K2O >2), low to moderate Rb (generally<100 ppm), moderate to high Sr (200–1500 ppm), enriched light rare earth elements (LREE) (CeN generally 10–150), fractionated REE (CeN/YbN 8–30), Eu anomaly (Eu/Eu*) 1, and decreasing REE with increasing SiO2. The plutons all contain amphibole-rich, mafic-ultramafic rocks which occur as enclaves and igneous layers and as intrusive units which exhibit textures indicative of contemporaneous mafic and felsic magmatism. Mafic mineral assemblages include: hornblende + biotite in tonalites; augite + biotite ± orthopyroxene ± pargasitic hornblende or hornblende+biotite in dioritic to monzodioritic rocks; and aegirine-augite ± silicic edenite ± biotite in syenite to alkali granite. Discrete plagioclase and microcline grains are present in most of the suites, however, some of the syenitic rocks are hypersolvus granitoids and contain only perthite. Mafic-ultramafic rocks have REE and Y contents indicative of their formation as amphibole-rich cumulates from the associated granitoids. Some cumulate rocks have skeletal amphibole with XMg(Mg/(Mg+ Fe2+)) indicative of crystallization from more primitive liquids than the host granitoids. Geochemical variation in the granitoid suites is compatible with fractionation of amphibole together with subordinate plagioclase and, in some cases, mixing of fractionated and primitive magmas. Mafic to ultramafic units with magnesium-rich cumulus phases and primitive granitoids (mol MgO/ (MgO+0.9 FeOTOTAL) from 0.60 to 0.70 and CT >150 ppm) are comagmatic with the evolved granitoids and indicate that the suites are mantle-derived. Isotopic studies of Archean monzodioritic rocks have shown LREE enrichment and initial 143Nd/144Nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements (LILE) shortly before melting. Mineral assemblages record lower PH2O with increased alkali contents of the suites. This evidence, in conjunction with experimental studies, suggests that increased alkali contents may reflect decreased PH2O during mantle melting. These features indicate that 2.73 Ga tonalitic rocks are derived from more hydrous mantle sources than 2.68 Ga syenitic rocks, and that the spectrum of late Archean juvenile granitoid rocks is broader than previously recognized. Comparison with Phanerozoic and recent plutonic suites suggests that these Archean suites are subduction related.  相似文献   

14.
《Gondwana Research》2014,26(4):1570-1598
Granitic rocks are commonly used as means to study chemical evolution of continental crust, particularly, their isotopic compositions, which reflect the relative contributions of mantle and crustal components in their genesis. New SIMS and K–Ar geochronology, isotope, geochemical, and mineral chemistry data are presented for the granitoid rocks located in and around Gabal Dara in the Northern Eastern Desert of Egypt. The granitoid suite comprises quartz diorites, Muscovite (Mus) trondhjemites, and granodiorites intruded by biotite-hornblende (BH) granites and alkali feldspar (AF) granites. Mus trondhjemite, granodiorite and BH granite exhibit I-type calc alkaline affinities. Mus trondhjemite and granodiorite show medium-K calc-alkaline and metaluminous/mildy peraluminous affinities, whereas BH granites have high-K calc-alkaline and metaluminous character. Concordant 206Pb/238U weighted mean ages together with geochemical peculiarities suggest that Mus trondhjemites (741 Ma) followed by granodiorites (720 Ma) are genetically unrelated, and formed in subduction-related regime by partial melting of lower oceanic crust together with a significant proportion of mantle melt. The genesis of Mus trondhjemites is correlated with the main event in the evolution of the Eastern Desert, called “~750 Ma crust forming event”.The field and geochemical criteria together with age data assign the high-K calc-alkaline BH granites (608–590 Ma) and alkaline AF granites (600–592 Ma) as post-collisional granites. The differences in geochemical traits, e.g. high-K calc-alkaline versus alkaline/peralkaline affinities respectively, suggest that BH granites and AF granites are genetically unrelated. The age overlap indicating coeval generation of calc-alkaline and alkaline melts, which in turn suggests that magma genesis was controlled by local composition of the source. The high-K calc-alkaline BH granites are most likely generated from lithospheric mantle melt which have been hybridized by crustal melts produced by underplating process. AF granites exhibit enrichment in K2O, Rb, Nb, Y, and Th, and depletion in Al2O3, TiO2, MgO, CaO, FeO, P2O5, Sr, and Ba as well as alkaline/peralkaline affinity. These geochemical criteria combined with the moderately fractionated rare earth elements pattern (LaN/YbN = 9–14) suggest that AF granite magma might have been generated by partial melting of Arabian–Nubian Shield (ANS) arc crust in response of upwelling of hot asthenospheric mantle melts, which became in direct contact with lower ANS continental crust material due to delamination. Furthermore, a minor role of crystal fractionation of plagioclase, amphibole, biotite, zircon, and titanomagnetite in the evolution of AF granites is also suggested. The low initial 87Sr/86Sr ratios (0.7033–0.7037) and positive εNd(T) values (+ 2.32 to + 4.71) clearly reflect a significant involvement of depleted mantle source in the generation of the post-collision granites and a juvenile nature for the ANS.  相似文献   

15.
Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO2 (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO2, P2O5, MnO, Al2O3 and Fe2O3 define linear trends with negative slopes against SiO2, whilst Na2O and K2O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO2, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (LaN/YbN = 4.5–6.4) with moderate HREE fractionation (SmN/YbN = 1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (< 400 ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the 87Sr/86Sr initial ratios vary from 0.704432 to 0.705989, and the 143Nd/144Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks.  相似文献   

16.
The Ziyang area is one of the two major regions of central China subjected to selenium (Se) poisoning. Systematic studies of Se contents of different lithologies from this area indicate that Lower Cambrian, carbonaceous, and siliceous strata host the highest Se contents (with Se contents of up to 278 ppm). We have investigated their geochemical characteristics (major and trace elements, and Si and O isotopes), origin, and sedimentary environment of formation. The siliceous rocks are characterized by a wide range in major elements, and are enriched in Se, Ba, Cu, Ni, V, As, Sb, and U relative to average continental crust. They are also enriched in light rare earth elements relative to heavy rare earth elements (LaN/YbN?=?1.64–35.7) and show weak or moderate negative Ce anomalies and strong positive Eu anomalies. δ30SiNBS-28 and δ18OV-SMOW values range from –0.3‰ to 0.6‰ and 16.1‰ to 21.7‰, respectively. The homogenization temperatures of inclusions within the studied samples range from 113°C to 319°C, and their salinities from 1.2 to 13.7 wt.% NaCl equivalent. Our results suggest that the studied siliceous rocks resulted from hydrothermal sedimentation in a relatively anoxic semi-deep sea sedimentary environment. The hydrothermal fluid responsible for Se-mineralization involved the mixing of low-temperature high-salinity fluid, low-temperature low-salinity fluid, and a high-temperature low-salinity basinal fluid in the NaCl–(KCl)–H2O system.  相似文献   

17.
The Sirstan granitoid (SG), comprising diorite and granodiorite, is located in the Shalair Valley area, in the northeastern part of Iraq within the Sanandaj–Sirjan Zone (SSZ) of the Zagros Orogenic Belt. The U–Pb zircon dating of the SG rocks has revealed a concordia age of 110 Ma, which is interpreted as the age of crystallization of this granitoid body during the Middle Cretaceous. The whole-rock Rb–Sr isochron data shows an age of 52.4 ± 9.4 Ma (MSWD = 1.7), which implies the reactivation of the granitoid body in the Early Eocene due to the collision between the Arabian and Iranian plates. These rocks show metaluminous affinity with low values of Nb, Ta and Ti compared to chondrite, suggesting the generation of these rocks over the subduction zone in an active continental margin regime. The SG rocks are hornblende-bearing I-type granitoids with microgranular mafic enclaves. The positive values of ?Nd (t = 110 Ma) (+0.1 to +2.7) and the low (87Sr/86Sr)i ratios (0.7044 to 0.7057) indicate that the magma source of the SG granitoids is a depleted subcontinental mantle. The chemical and isotope compositions show that the SG body originated from the metasomatic mantle without a major role for continental contamination. Our findings show that the granitoid bodies distributed in the SSZ were derived from the continuous Neo-Tethys subduction beneath the SSZ in Mesozoic times and that the SSZ was an active margin in the Middle Cretaceous.  相似文献   

18.
《International Geology Review》2012,54(11):1401-1417
The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO2 (38–52 wt.%) and high Mg# (～48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ? Nd(t) = 5.3–8.5 and 147Sm/144Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.  相似文献   

19.
ABSTRACT

Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO2 = 67.5–69.4 wt%, high K2O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd(t) (?3 to ?2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO2 (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd(t) (?2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.  相似文献   

20.
The Eocene volcano-sedimentary units in the southern part of the Eastern Pontides (NE Turkey) are confined within a narrow zone of east–west trending, semi-isolated basins in Bayburt, Gümü?hane, ?iran and Alucra areas. The volcanic rocks in these areas are mainly basalt and andesite through dacite, with a dominant calc-alkaline to rare tholeiitic tendency. 40Ar–39Ar dating of these volcanic rocks places them between 37.7 ± 0.2 and 44.5 ± 0.2 Ma (Middle Eocene). Differences in the major and trace element variations can be explained by the fractionation of clinopyroxene ± magnetite in basaltic rocks and that of hornblende + plagioclase ± magnetite ± apatite in andesitic rocks. Primitive mantle-normalized multi-element variations exhibit enrichment of large-ion lithophile elements and to a lesser extent, of light rare earth elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magma derived from an enriched mantle source. Chondrite-normalized rare earth element patterns of the aforementioned volcanic rocks resemble each other and are spoon-shaped with low-to-medium enrichment (LaN/LuN = 2–14), indicating similar spinel lherzolitic mantle source(s). Sr, Nd and Pb isotopic systematics imply that the volcanic rocks are derived from a subduction-modified subcontinental lithospheric mantle. Furthermore, post-collisional thickened continental crust, lithospheric delamination and a subduction-imposed thermal structure are very important in generating Tertiary magma(s). The predominantly calc-alkaline nature of Eocene volcanic rocks is associated with increasing geodynamic regime-extension, whereas tholeiitic volcanism results from local variations in the stress regime of the ongoing extension and the thermal structure, as well as the thickness of the crust and the mantle-crust source regions. Based on volcanic variety and distribution, as well as on petrological data, Tertiary magmatic activity in Eastern Pontides is closely related to post-collisional thinning of the young lithosphere, which, in turn, is caused by extension and lithospheric delamination after collisional events between the Tauride–Anatolide Platform and the Eurasian Plate.  相似文献   

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