Diorite Dykes Research Articles

Positioning the Southern Margin of Asia Prior to Its Collision with India: Paleomagnetic Constraints from Late Cretaceous Dykes in Gangdise Belt

The pre‐collisional southern margin of Asia can be restored using paleomagnetic data from late Cretaceous rocks from the Lhasa terrane. However, the available data are based either on the red beds or on the intercalated thin layers of lava flows, both of which had been involved in strongly folding. Recent studies show clear evidence for the possibility of serious overprint hence the data could not be reliably used for tectonic interpretation. We report paleomagnetic data from diorite dykes and the grano‐diorite country rock in the Gandise belt near the city of Lhasa. U–Pb isotopic dating indicates the intrusive rocks have an age of ∼82–86 Ma. Fifteen sites yield acceptable ChRM directions which pass a reversal test. SEM and light microscope observations show primary intergrowth relationship between magnetite and other minerals within the thin sections. AMS measurement defines a primary magma flow fabric for the intruded dykes and the country rocks. All the characteristics support that the ChRMs are primary. The paleomagnetic pole calculated from the remanence of the dykes and the country rocks yields a paleolatitude of ∼14°N which provide a reliable constraint for the southern margin of Asia near Lhasa. Furthermore, the recorded declination shows significant counterclockwise rotation of ∼20° for the sampling location relative to the north. In consideration of the strike and tectonic setting of the dykes, the strike of the southern margin of Asia is restored which is compatible with the hypothesis of a quasi‐linear margin of Eurasia prior to its collision with India.

Ductile–brittle deformation effects on crystal-chemistry and U–Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea–Verbano Zone, Italian Alps)

A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea–Verbano Zone, Italy). The leucocratic dykes consist mainly of plagioclase (An18–24Ab79–82Or0.3–0.7) with subordinate amounts of biotite and spinel defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions.Zircon grains/fragments (up to 2mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinels. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals.Cathodoluminescence (CL) images reveal that zircon grains from the leucocratic layers show relicts of primary magmatic (i.e. oscillatory and or sector) zoning generally related with the crystal shapes or crystallographic orientation, whereas those from the melanocratic domains do not. In both cases, zircon shows secondary CL features, i.e. mosaic-like textures, due to deformation. EBSD maps confirmed a profuse mosaic texture, resulting in an internal misorientation of ca. 10°, generally associated with fractures. Locally, zircon shows clear evidence of crystal-plastic deformation at the edges, with a gradual misorientations of up to 12°, suggesting an origin prior fragmentation.Trace elements and U–Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections. Such in situ investigations revealed a strong correlation between internal zircon structures, chemistry, U–Pb isotope ratios and mylonitic fabric.U–Pb data return highly discordant and variable ages. The 206Pb/238U ages may range from ca. 297 to 198Ma within the same zircon grain from the leucocratic layers, whereas 206Pb/238U younger than 250Ma were systematically obtained from zircon within the melanocratic layers. The 206Pb/238U data younger than ca. 240Ma from zircon grains within the leucocratic layers were obtained from narrow axial stripes observed in CL images and oriented parallel or at low-angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases. Combining U–Pb data, microstructure and zircon CL features, we suggest a multistage evolution of the dykes whereby the melanocratic layers are the result of a Late Permian metasomatic event promoting modification of the pre-existing mineral assemblage of the (Late Carboniferous–Early Permian?) dykes and deformation and partial resetting of zircon porphyroclasts. This has important implications in the geology of the Ivrea–Verbano Zone because, having the dyke intruded the External Gabbro unit before Triassic (probably during Carboniferous–Permian), the latter cannot be considered a Triassic intrusion. At least part of the External Gabbro unit is Carboniferous–Permian as the other mafic bodies of the Ivrea–Verbano Zone.

Arc magmatism associated with steep subduction: Insights from trace element and Sr–Nd–Hf–B isotope systematics

AbstractSubduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318–312 Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction‐like geochemical signatures, together with relatively high Mg#, high εNd(t) and εHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction‐modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid‐ocean ridge basalt‐like Sr–Nd isotopes and high zircon εHf(t), similar to slab‐derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from −7.7 to −6.4‰, whereas the adakitic dykes have relatively high δ11B values from −6.9 to −4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B‐depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high‐angle dip of the subducting oceanic slab.

The intra-oceanic Cretaceous (~ 108 Ma) Kata–Rash arc fragment in the Kurdistan segment of Iraqi Zagros suture zone: Implications for Neotethys evolution and closure

The Kata–Rash arc fragment is an allochthonous thrust-bound body situated near Penjween, 100km northeast of Sulymannia city, Kurdistan Region, within the Iraqi portion of the Zagros suture zone. It forms part of the suprasubduction zone ‘Upper Allochthon’ terranes (designated as the Gimo–Qandil Group), which is dominated by calc-alkaline andesite and basaltic–andesite, rhyodacite to rhyolite, crosscut by granitic, granodioritic, and dioritic dykes. Previously, rocks of the Kata–Rash arc fragment were interpreted as a part of the Eocene Walash volcanic group. However, SHRIMP zircon U–Pb dates on them of 108.1±2.9Ma (Harbar volcanic rocks) and 107.7±1.9Ma (Aulan intrusion) indicate an Albian–Cenomanian age, which is interpreted as the time of igneous crystallisation. The Aulan intrusion zircons have initial εHf values of +8.6±0.2. On a Nb/Yb–Th/Yb diagram, all Kata–Rash samples fall within the compositional field of arc-related rocks, i.e. above the mid-ocean-ridge basalt (MORB)—ocean island basalt (OIB) mantle array. Primitive-mantle-normalised trace-element patterns for the Kata–Rash samples show enrichment in the large ion lithophile elements and depletion in the high-field-strength elements supporting their subduction-related character. Low Ba/La coupled with low La/Yb and Hf/Hf* <1 for the Aulan sample with initial εHf of +8.6±0.2 is interpreted as the magma dominated by contributions from fluid fluxing of the mantle wedge and lesser contributions of low temperature melt from subducted slab sediment, in an oceanic setting. This mechanism can explain the sub-DM initial εHf value, without the need to invoke melting of significantly older (continental) crust in an Andean setting. We interpret the Kata–Rash igneous rocks as a fragment of the Late Cretaceous suprasubduction zone system (named here the Kata–Rash arc) that most likely developed within the Neotethys Ocean rather than at a continental margin. Subsequently during the latest Cretaceous to Paleocene, the arc was accreted to the northern margin of the Arabian plate. The results indicate a >3000km continuity of Cretaceous arc activity (Oman to Cyprus), that consumed Neotethyian oceanic crust between Eurasia and the Gondwanan fragment Arabia.

The structural history and mineralization controls of the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The Geita Hill gold deposit is located in the Archean Geita Greenstone Belt and is one of the largest gold deposits in East Africa. The Geita Greenstone Belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the Geita Hill gold deposit. Deformation involved early stages of ductile shearing and folding (D1 to D5), during which episodic emplacement of large diorite intrusive complexes, sills, and dykes occurred. These ductile deformation phases were followed by the development of brittle-ductile shear zones and faults (D6 to D8). The last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes, within the greenstone belt, and the emplacement of large granitic bodies now forming the margins of the greenstone belt. Early, folded lamprophyre dykes, and later lamprophyre dykes, crosscutting the folded sequence are common, although volumetrically insignificant. The gold deposit formed late during the tectonic history of the greenstone belt, post-dating ductile deformation and synchronous with the development of brittle-ductile shear zones that overprinted earlier structural elements. The main mineralizing process involved sulfide replacement of magnetite-rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions. The intersection between the brittle-ductile (D6) Geita Hill Shear Zone and different structural elements of ductile origin (e.g., fold hinges), and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization.

The Paleoproterozoic diorite dykes in the southern margin of the North China Craton: Insight into rift-related magmatism

The Xiong’er rift zone along the southern margin of the North China Craton (NCC) is characterized by exposure of a suite of volcanic rocks and associated mafic – intermediate dykes and sedimentary rocks. In this study, we report for the first time the newly identified late Paleoproterozoic diorite dykes from the Waifangshan Mountain in the Xiong’er rift zone and present results from the LA-ICP-MS zircon U–Pb dating, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element geochemistry. The Wafang diorite dykes intruding into the upper part of the Xiong’er Group yielded emplacement mean ages of 207Pb/206Pb from 1746±22Ma to 1762±11Ma. These diorite dykes have extreme low MgO, Cr and Ni contents, and enriched in LREEs and LILEs but depleted in HFSEs (Nb, Ta, and Ti). They are characterized by having negative zircon ɛHf(t) values of −11.4 to −3.1, a whole-rock initial 87Sr/86Sr value of 0.70196 and ɛNd(t) value of −8.7, and Pb isotopes (206Pb/204Pb=16.072–16.295, 207Pb/204Pb=15.275–15.293, 208Pb/204Pb=36.538–37.255). These geochemical features, together with trace element modeling, suggest that the late Paleoproterozoic Wafang diorite dykes could be sourced from crustal melting with minor contribution of the mixing of mantle materials. These new data combined with previous studies on the late Paleoproterozoic the Xiong’er volcanism should have lasted for nearly 43Ma ranging from 1789Ma to 1746Ma. Correspondingly, the initiation timing of the Xiong’er rift should be younger than 1831Ma, but earlier than 1625. The Wafang diorite dykes formed in a rift environment, possibly related to breakup of the supercontinent Columbia.

Eocene slab breakoff of Neotethys as suggested by dioritic dykes in the Gangdese magmatic belt, southern Tibet

The Gangdese magmatic belt in southern Tibet demarcates an important boundary between the Indian and Eurasian plates. Due to its location and magmatic evolutionary history, it is key to understanding both the history of Neotethys closure and the Indo–Asian collisional process. This study presents new geochronological and geochemical data for dioritic dykes in the southern Gangdese magmatic belt in southern Tibet. U-Pb geochronological results reveal that the dykes were emplaced at ca. 41Ma and thus broadly coeval with the 40–38Ma Dazi volcanics and the 42–40Ma Gaoligong–Tengliang basaltic dykes. Geochemically, these dykes are characterized by alkaline signature, high Mg# (57–63) and low TiO2 contents (~0.9–1.0), showing notable enrichment of light rare earth elements relative to the heavy rare earth elements, enrichment of incompatible elements (i.e. Cs, Rb, Ba, Th and U), and depletion of high field strength elements (i.e. Nb, Ta and Ti). In addition, a large variation of zircon εHf(t) values (−10 to +13) was shown, implying heterogeneity of magma sources. A heterogeneous source is also suggested by the occurrence of xenocrysts in the dykes. These observations suggest that the magma source of the dykes was dominated by partial melting of lithospheric mantle and then subsequently contaminated by crustal material during ascent. In combination with other geological data in the region, we suspect that the slab slicing of the Neotethys played a key role in the formation of the lithospheric mantle-derived dioritic dykes and adakitic granite, asthenosphere-derived volcanics, basaltic dykes, as well as the recently reported strongly fractionated granites.

Two Periods of Skarn Mineralization in theBaizhangyanW–MoDeposit, SouthernAnhui Province, SoutheastChina: Evidence from ZirconU–Pband MolybdeniteRe–Osand Sulfur Isotope Data

AbstractThe recently discoveredBaizhangyan skarn‐porphyry typeW–Modeposit in southernAnhui Province inSE China occurs near theMiddle–LowerYangtzeValley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with theMesozoicQingyang granitic complex composed ofgranodiorite, monzoniticgranite, and alkalinegranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones ofSinianLantianFormation contacting with buried fine‐grained granite, and diorite dykes. There are two types ofWmineralization: major skarnW–Momineralization and minor granite‐hosted disseminatedMomineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the firstW–Auperiod related to the intrusion of diorite dykes, and the subsequentW–Moperiod related to the intrusion of the fine‐grained granite. In this paper, we report new zirconU–Pband molybdeniteRe–Osages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, andWmineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted meanU–Pbages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdeniteRe–Osage determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ34Svalues of 3.6‰–6.6‰ and theirRecontents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containingReconcentrations of 50.5–50.9 ppm, and with δ34S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations ofRe(7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarnW–Aumineralization took place in theEarlyCretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarnW–Momineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of theQingyang monzonitic granite. This finding suggests that the first period of skarnW–Aumineralization in theBaizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period ofW–Momineralization derived from the crust at 135.0–136.9 Ma.

Fractional crystallization and magma mixing: evidence from porphyritic diorite-granodiorite dykes and mafic microgranular enclaves within the Zhoukoudian pluton, Beijing

The Zhoukoudian pluton in the North China craton is a circular granodiorite intrusion containing porphyritic diorite dykes (PDDs), porphyritic granodiorite dykes (PGDs) and abundant mafic microgranular enclaves (MMEs), which provide an excellent opportunity to study fractional crystallization and magma mixing. The PDDs and PGDs are located in the western part of the pluton with the PDDs intruded by the PGDs. The dykes have similar mineral assemblages although plagioclase in the PDDs has higher anorthite content than the PGDs. Linear relationships between the SiO2 and most major and trace element contents, as well as a positive trend of initial 87Sr/86Sr ratios and a negative trend of epsilon Nd values with increasing SiO2 contents for the dykes suggest that both types were formed by assimilation and fractional crystallization of a common parental magma. Major oxide mass balance and trace element Rayleigh fractionation modeling points to early separation of garnet (11 %), clinopyroxene (27 %), orthopyroxene (16 %), plagioclase (25 %), biotite (19 %), and apatite (2 %) and late fractionation of hornblende (25 %), plagioclase (46 %), biotite (25 %), apatite (1 %), and magnetite (3 %). Most MMEs occur within the transitional granodiorite of the Zhoukoudian pluton. Zoned MMEs, dyke-like MME swarms, local presence of concave margins, veins and enclaves of host granodiorite within some MMEs, and several MMEs surrounded by the biotite-rich granodiorite support their formation by multiple magma mixing events, which finally resulted in different whole-rock major oxides and compatible elements, but homogeneous mineral major oxides (except zoned plagioclase), whole-rock incompatible elements and Sr-Nd isotopes between the MMEs and their host granodiorite. We suggest that multiple magma mixing events might also cause complexly zoned plagioclase in the Zhoukoudian pluton. Relative calcic, irregular or patchy cores and dusty zoned mantles from the zoned plagioclase crystals and their relatively low anorthite content indicate multiple mixing events between mafic/intermediate and felsic magmas. The mafic/intermediate end-members could be represented by the diabase dykes and the PDDs. Therefore, the dykes and MMEs in the Zhoukoudian pluton are genetically linked.

Petrogenesis of late Paleozoic tholeiitic, Nb-enriched, calc-alkaline and adakitic rocks in southwestern Mongolia: Implications for intra-oceanic arc evolution

Abstract Intra-oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense Central Asian Orogenic Belt (CAOB), however, the detailed geological processes of the development and growth of numerous island arcs still remain unclear. In order to explore the development history of representative ancient island arcs, we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern Mongolia. The studied rock association comprises a mafic intrusive complex (consisting of norite and diabase-porphyry), diabase and diorite dykes. New zircon geochronological data and whole-rock geochemical compositions reveal sequential magmatic events of an island arc development. The mafic intrusive complex has an emplacement age of 356 Ma old. Geochemically, its two components of norite and diabase-porphyry have a common “tholeiitic” character, but show striking differences in elemental contents. The norites are characterized by relatively higher MgO (7.25–16.1 wt.%), lower TiO 2 (0.54–0.93 wt.%) and relatively lower Nb contents (0.99–6.39 ppm) than those of the Nb-enriched (Nb > 9 ppm) diabase-porphyry. These characters, together with variable Nb/La ratios, suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab-derived fluids and/or melts, whereas, the generation of the Nb-enriched diabase-porphyry was associated with the mantle metasomatism by slab-derived melts. The 335 Ma diabase dykes have negative Nb–Ta–Ti anomalies and elevated LREE and LILE contents, similar to calc-alkaline subduction-related magmas, which together with high zircon e Hf (t) values (+ 10.6 to + 15.8) indicate a derivation from a mantle wedge metasomatized by slab-related fluids. The 290 Ma dioritic dykes show the highest SiO 2 (53.1–58.3 wt.%) and total alkali (K 2 O + Na 2 O = 5.84–7.77 wt.%) contents, and their distinctively high Sr/Y (56.1–97.9) ratios and low Yb ( The timing and petrogenesis of the basic–intermediate rock association may supply valuable information on tectonic evolution of an intra-oceanic subduction–accretion system of the CAOB. The coexistence of the early Carboniferous tholeiitic, Nb-enriched mafic intrusive complex, and boninite as well as picrite may represent an immature intra-oceanic island arc. The middle Carboniferous calc-alkaline diabase dykes were probably associated with the relatively mature island arc. The Permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime, when numerous subduction–accretion complexes were likely amalgamated together and docked on the Siberian active margin.

Geochronology, geochemistry and Hf isotope of Late Triassic magmatic rocks of Qingchengzi district in Liaodong peninsula, Northeast China

The initiation timing and mechanism of lithospheric thinning of the North China Craton (NCC) was still controversial. Late Triassic igneous rocks especially mantle derived mafic rocks would provide constrains on Early Mesozoic lithospheric mantle geodynamics and initiation of lithospheric thinning. This paper reports Late Triassic magmatic rocks, including lamprophyre, diorite dykes and biotite monzogranite cropped out in Qingchengzi district of Liaodong peninsula, northeastern NCC. LA–ICPMS zircon U–Pb dating yield ages of 210–227Ma and 224Ma for lamprophyres and biotite monzogranite respectively. Lamprophyre is ultrapotassic, strongly enriched in REE and LILEs, depleted in HFSEs, and negative Hf isotopes, which are discriminating signatures of crustal source, but distinguishingly high compatible element contents indicate the primary magma originated from mantle source—a fertile one. Lamprophyre derived from partial melting of an enriched lithospheric mantle, which was modified by slab-derived hydrous fluids/melts associated with deep subduction between the Yangtze Craton and the NCC. The diorite displays distinct features with relatively enriched Nb, Ta, HREE and depleted Th, U, which suggest it derived from a relatively depleted source. The depletion was caused by break-off of the Yangtze slab during deep subduction introducing asthenospheric mantle into the source. The biotite monzogranite shows adakitic affinity, and originated from partial melting of the thickened lower crust with addition of small proportion of mantle material. The recognition of Late Triassic magmatism implies extensional tectonic settings in Liaodong peninsula and suggests initiation of lithospheric thinning of North China Craton in eastern segment might begin early in Late Triassic.

Geochemistry of island arc dolerites and diorites along Qift–Quseir asphaltic road, central Eastern Desert, Egypt

The Precambrian basement rocks exposed along Qift–Quseir asphaltic road, central Eastern Desert of Egypt, exhibit two contrasted tectonic units, each of which has its own lithology structural style and grade of metamorphism. They are intruded by dolerite and diorite dykes. The alkali (Na2O+K2O) and TiO2 contents increase whereas Al2O3, FeO, MgO, CaO and MnO decrease with increasing SiO2 from dolerites to diorites. The trace elements Ti, Zr, Cr, Y and Ni indicate that the dolerites are tholeiitic with slight tendency toward calc-alkaline and formed from basaltic magma in an active continental margin, while diorites are calc-alkaline and were formed by fractional crystallisation of high-alumina basaltic magma in an island arc and active continental margin tectonic environment where they probably represent the forerunner of G1 granites. The molecular ratios Mg values (MgO×100/MgO+FeO) of dolerites range from 47 to 49 while those of diorite range from 51 to 59, indicating that the dolerite and diorite have suffered mild fractionation. Mineral chemistry for the diorites shows that the amphiboles are classified as magnesiohornblende and the plagioclase composition is An39–42 (i.e. the narrow range indicate that the pluton has not suffered extensive fractional crystallisation). The Al content of amphibole displays significant variation with pressure and temperature, also the change of the Ca/(Ca+Na) ratio of plagioclase is dependent on temperature. The amphibole–plagioclase geothermobarometer suggested the P-T formation conditions of studied dykes as 2 kbar and 600 °C.

Tracing a subducted ridge–transform system in a late Carboniferous accretionary prism of the southern Altaids: Orthogonal sanukitoid dyke swarms in Western Junggar, NW China

Two sets of Late Paleozoic, orthogonal, dioritic dyke swarms are defined in the eastern part of the Western Junggar accretionary collage in the southern Altaids. Dioritic dykes (hornblende+plagioclase) are characterized by relatively high SiO2, elevated MgO, Na2O, K2O, Mg#, Ni, Si and Cr contents, La/Yb ratios, very low Y contents, and enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE), which are all comparable with those of sanukitoids. These dioritic dykes have low (87Sr/86Sr) ratios (mostly between 0.70366 and 0.70381) and high positive εNd (t) (+6.6 to +8.4) values, suggesting they were sourced from a depleted mantle. Accordingly, these dioritic dykes were probably derived by mixing of depleted mantle components that originated from upwelling of asthenosphere through a slab window with melts that were derived by dehydration of a subducted slab. Detailed mapping and structural data of the dyke swarms indicate that the NW/SE-trending set was earlier than the NE/SW-trending dykes. Paleostress analysis and inversion of the dyke swarms indicate that an early NE/SW-extension was associated with a NW/SE-trending slab window, and was followed by a slightly later NW/SE-extension that was associated with a NE/SW-trending slab window. Field relationships, together with published age constraints, suggest that these two dyke swarms were intruded in the Late Carboniferous. Therefore two orthogonal slab windows were opened, one (NE-SW) after the other (NW-SE) during this time period. A subducted ridge–transform system could well account for, and is consistent with, these relationships. We propose that in the Late Carboniferous a NW/SE-trending mid-oceanic ridge was subducted beneath the Darbut trench generating large-scale, closely spaced NW/SE-trending dykes that were intruded through a slab window. That event was followed by subduction of a NE/SW-trending transform fault (fracture zone) that connected with the earlier NW/SE-trending ridge, producing widely spaced NE/SW-trending dykes that were intruded through a NE/SW-trending vertical slab window. This new approach to the relationships between dyke swarms, adakitic/sanukitoid magmatism, ridge subduction and slab windows has broad implications for tectonic reconstruction of ancient accretionary orogens.

Structural and geochronological studies on the Liba goldfield of the West Qinling Orogen, Central China

The Liba goldfield, located to the northeast of the Zhongchuan Granite in the West Qinling Orogen (WQO) of mainland China, contains the largest known gold resource of 2.8 Moz in the Zhongchuan area. Devonian metasedimentary rocks host the structurally controlled gold mineralization, which is associated with silica–sericite–chlorite–carbonate alteration. Two major styles of mineralization occurred at the goldfield, which are disseminated sediment-hosted and quartz vein hosted types. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides. Numerous felsic/intermediate dykes have a similar structural control as the mineralization, and their contacts with host rocks are recognized as favorable zones for mineralization. Detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield. Three major phases of deformation have been recognized in the Zhongchuan area. The first deformation (D1) event was compressional in broadly a N–S orientation, the second (D2) event was also compressional and orientated in a NE–SW direction, and the third (D3) event was post-mineralization and was associated with the emplacement of barren calcite and anhydrite veins. Compression related to D2 is the key process that controlled the distribution of igneous dykes and gold mineralization in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans-extension, which took place during the late stage of D2. Precise dating with high-resolution ion microprobe (SHRIMP) U–Pb on zircon and 40Ar/39Ar on muscovite, biotite, hornblende, and plagioclase of crosscutting pre-mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca. 227 Ma. 40Ar/39Ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralization indicates that there was a widespread ca. 216 Ma hydrothermal event that affected almost all lithologies in the area. This detailed investigation is the first study to tightly constrain the timing of gold mineralization in the WQO. The broadly overlapping timing and similar structural control of the mineralization and igneous dykes show a promising correlation, which could be potentially used to map this Late Triassic gold mineralization event in the WQO.

Pyrrhotite remagnetizations in the Himalaya: a review

Abstract This paper reviews results on the nature and thermo-tectonic interpretation of widespread pyrrhotite remagnetizations in the Himalaya. Throughout the last two decades a large dataset has been acquired, in particular from low-grade metamorphic rocks of the Tethyan Sedimentary Series (TSS). The nature of this magnetization is a thermoremanence when the peak metamorphic temperature T max exceeded the Curie temperature ( T c c. 325 °C) of pyrrhotite; in this case the remanence age can be related to last metamorphic cooling. For T max &lt; T c , the remanence can be of chemical, thermochemical or thermoremanent origin. Cooling ages show a systematic trend of c. 50–20 Ma from the western to the eastern Himalaya. The pyrrhotite remagnetizations post-date main Himalayan folding and record late orogenic long-wavelength rotations and tiltings around vertical and horizontal axes. Remanence directions in the western Himalaya are well matching with large-scale deformations of rotational shortening and oroclinal bending, while in the central and eastern Himalaya they are predominantly controlled by meso-scale effects due to crustal doming. Stable pyrrhotite remanences are especially typical for low-grade marly limestones in the TSS, but were also found in medium-grade rocks of the Lesser Himalaya, highly metamorphic rocks of the Higher Himalayan Crystalline and diorite dykes intruding into the TSS.

청주 화강암의 SHRIMP 저어콘 U-Pb 연대, 지구화학 및 Sr-Nd 동위원소 연구

The emplacement ages, whole-rock geochemistry and Sr-Nd isotopic compositions of granitoid rocks from Cheongju area, South Korea, were investigated for delineating their petrogenetic link to the Jurassic Daebo granitoid rocks. Zircon crystals were collected from the diorite, biotite granite and acidic dyke samples in a single outcrop. Cross-cutting relationships show that the emplacement of diorite was postdated by the intrusion of biotite granite. Both rocks have been subsequently intruded by acidic dyke. The U-Pb isotopic compositions of zircon from the diorite, biotite granite, and acidic dyke were measured using a SHRIMP-II ion microprobe, yielding the crystallization ages of , , and , respectively, with 95% confidence limits (). The emplacement ages are consistent with those determined from the above relative ages. The major and trace element patterns of the rocks are consistent with those of the Jurassic Daebo granitoid rocks, possibly suggesting a subduction-related I-type granite. The geochemical signature is, however, betrayed by the Sr and Nd isotopic compositions of these rocks. The isotopic signatures suggest that the rocks were produced either by the partial melting of lower-crust or by the mantle-derived magma contaminated by the basement rocks during its ascent and/or emplacement. In addition, the inherited ages of zircons of the rocks (ca. 2.1, 1.8, 0.8 and 0.4 Ga) suggest a possible assimilation with crustal rocks from the Gyeonggi massif and Ogcheon metamorphic belt.

An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous: Evidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarzê, Southern Tibet

Abstract Widely distributed in Gyangze-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase(gabbro)-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarze were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean 206Pb/238U ages were obtained in this paper, which are 134.9±1.8 Ma (MSWD = 0.65) and 135.5 ± 2.1 Ma (MSWD=1.40), respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.

Age and geochemistry of mantle peridotites and diorite dykes from the Baldissero body: Insights into the Paleozoic–Mesozoic evolution of the Southern Alps

Trace element and isotopic data obtained for mantle spinel lherzolites and diorite dykes from the Baldissero massif (Ivrea–Verbano Zone, Western Italy) provide new, valuable constraints on the petrologic and geodynamic evolution of the Southern Alps in Paleozoic to Mesozoic times. Whole rock and mineral chemistry indicates that Baldissero lherzolites can be regarded as refractory mantle residues following limited melt extraction. In particular, the Light Rare Earth Elements (LREE)-depleted and fractionated compositions of whole rock and clinopyroxene closely match modelling results for refractory residues after low degrees (~ 4–5%) of near-fractional melting of depleted mantle, possibly under garnet-facies conditions. Following this, the peridotite sequence experienced subsolidus re-equilibration at lithospheric spinel-facies conditions and intrusion of several generations of dykes. However, lherzolites far from dykes show very modest metasomatic changes, as evidenced by the crystallisation of accessory titanian pargasite and the occurrence of very slight enrichments in highly incompatible trace elements (e.g. Nb). The Re–Os data for lherzolites far from the dykes yield a 376 Ma (Upper Devonian) model age that is considered to record a partial melting event related to the Variscan orogenic cycle s.l. Dioritic dykes cutting the mantle sequence have whole rock, clinopyroxene and plagioclase characterised by high radiogenic Nd and low radiogenic Sr, which point to a depleted to slightly enriched mantle source. Whole rock and mafic phases of diorites have high Mg# values that positively correlate with the incompatible trace element concentrations. The peridotite at the dyke contact is enriched in orthopyroxene, iron and incompatible trace elements with respect to the lherzolites far from dykes. Numerical simulations indicate that the geochemical characteristics of the diorites can be explained by flow of a hydrous, silica-saturated melt accompanied by reaction with the ambient peridotite and fractional crystallisation. The composition of the more primitive melts calculated in equilibrium with the diorite minerals show tholeiitic to transitional affinity. Internal Sm–Nd, three-point isochrons obtained for two dykes suggest an Upper Triassic-Lower Jurassic emplacement age (from 204 ± 31 to 198 ± 29 Ma). Mesozoic igneous events are unknown in the southern Ivrea–Verbano Zone (IVZ), but the intrusion of hydrous melts, mostly silica-saturated, have been well documented in the Finero region, i.e. the northernmost part of IVZ and Triassic magmatism with calc-alkaline to shoshonitic affinity is abundant throughout the Central–Eastern Alps. The geochemical and chronological features of the Baldissero diorites shed new light on the geodynamic evolution of the Southern Alps before the opening of the Jurassic Tethys.

Paleomagnetic evidence for clockwise rotation and tilting in the eastern Tethyan Himalaya (SE Tibet): Implications for the Miocene tectonic evolution of the NE Himalaya

Crustal movement around and away from the Namche Barwa syntaxis is indicated in the Asian velocity field inferred from GPS data and Quaternary fault slip rates. Nevertheless, there is a limited field-based control on the rotational history of the north-eastern Himalayan arc. Exploring the poly-phase nature of deformation, within the Cretaceous diorite dykes and their host-rock (Triassic flysch), in the eastern Tethyan Himalaya (90°–92°E), combined with new remote sensing data and existing thermo-geochronological data, allow us to unravel the kinematic relationship between paleomagnetic remanence vectors and the deformation phases. Decay at 325 °C in high temperature susceptibility curves and in the thermal demagnetization of the SIRM indicate that the characteristic remanent magnetization in the Cretaceous diorite dykes is carried by pyrrhotite. The pyrrhotite component unblocks at 280–350 °C, revealing normal and reverse polarities. It is of post-folding origin with a mean remanence direction of 019°/28° and 001°/20° in the eastern (Qonggyai valley) and western (Nagarze) part of the sampling area, respectively. The ~ 22 Ma K–Ar age of the last metamorphic event support that the remanence is post-Eohimalayan folding and likely of thermoremanent or thermo-chemical origin. Comparison of the declination with respect to the Early-Miocene reference direction, yields a trend from no apparent rotation in the west to 20° clockwise rotation in the east with respect to the stable Indian plate. This result can be kinematically related to the Middle to Late-Miocene strain partitioning between far-field southeast extrusion of SE Tibet and near-field strike-slip faulting and E–W extension. Furthermore the observed pattern of tilting around horizontal axis may reflect concealed North Himalayan doming.

The origin of geochemical trends and Eoarchean (ca. 3700 Ma) zircons in Mesoarchean (ca. 3075 Ma) ocelli-hosting pillow basalts, Ivisaartoq greenstone belt, SW Greenland: Evidence for crustal contamination versus crustal recycling

The Mesoarchean Ivisaartoq greenstone belt consists of tholeiitic pillow basalts, picrites, boninites, gabbroic to dioritic dykes and sills, actinolite schists and serpentinites. In addition, the belt includes volumetrically minor siliceous volcaniclastic sedimentary rocks, cherts, biotite schists, and quartzitic gneisses. Siliceous volcaniclastic rocks yield an average U–Pb zircon age of 3075 ± 15 Ma, and pillow basalts and gabbros yield a Sm–Nd errorchron age of 3080 ± 160 Ma, constraining the maximum age of the belt. The Ivisaartoq rocks underwent high-temperature seafloor hydrothermal alteration, resulting in extensive epidote-diopside calc-silicate metasomatism. Th–REE–Nb systematics of the least metasomatized volcanic rocks are consistent with a supra-subduction zone geodynamic setting, displaying a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Many pillow basalts in the Ivisaartoq greenstone belt contain numerous millimetre- to centimetre-long white polycrystalline ellipsoidal inclusions, called ‘ocelli’, set in a dark green mafic matrix (basaltic host). On the basis of new major and trace element, Nd isotope, and petrographic data, we re-interpret the origin of these inclusions and suggest that they were derived from the melting of hydrated mafic to ultramafic rocks in the lower oceanic crust during magma–crust interaction. Some pillow basalts contain rare small inherited magmatic zircons revealing Eoarchean (3706 ± 8 to 3717 ± 4 Ma) 207Pb 206Pb ages. In the least metasomatized pillow basalts, and gabbros and diorites there is a strong negative correlation between initial Nd isotopic composition ( ε Nd = + 0.76 to + 3.10) and depleted mantle model ages ( T DM = 3100–3800 Ma). Post-magmatic metasomatic alteration can be ruled out as the cause of this negative correlation, suggesting the contamination of the Ivisaartoq rocks, or their mantle source, by Eoarchean (~ 3700 Ma) continental crust or sediments derived from such crust. Four possible processes of crustal contamination might have taken place during or prior to the emplacement of the Ivisaartoq belt: (1) interaction with continental crust during magma ascent; (2) mixing between the depleted upper mantle and recycled sediments derived from Eoarchean continental crust; (3) mixing between the depleted upper mantle and delaminated lower continental crust; and (4) assimilation of continentally-derived sediments during eruption. It is unlikely that recycled Eoarchean zircons and their host rocks could have survived through multiple events of partial melting in the Mesoarchean upper mantle. A tectonic slice of metasedimentary rocks exposed between the Ivisaartoq belt and the Eoarchean (3600–3870 Ma) Isukasia terrane contains magmatic zircon grains up to 3800 Ma old. Accordingly, we argue that Eoarchean zircons are likely to have been derived from the protoliths of these metasedimentary rocks. The negative correlation between initial ε Nd and T DM in pillow basalts, gabbros and diorites, however, can be better explained by contamination of the mantle source through hydrous melts and/or fluids, derived from Mesoarchean subducted (recycled) sediments, than by contamination at the surface.