Gabbro Diorite Research Articles

Late Mesozoic diorites of the middle Gangdese magmatic belt of southern Tibet: New insights from SHRIMP U-Pb dating and Sr-Nd-Hf-O isotopes

The Gangdese magmatic belt, which formed by Mesozoic northward subduction of Neo-Tethyan oceanic lithosphere and Early Mesozoic India-Asia collision, is an excellent natural laboratory for study of subducting slabs, crustal growth, the reworking of convergent continental margins and interaction of crust-mantle in general. Although many studies have examined this belt, the petrogenesis of arc magmas and the geodynamic architecture of the subducting slab are still contentious. Discoveries of igneous rock exposures are important in reconstructing the tectonic regime and evolution of southern Tibet, and provide new constraints on unresolved issues. Here we report two newly identified intermediate rocks, shedding new light on both the formation of arc-type magmas, and the tectonic evolution of the Neo-Tethys during the Late Mesozoic. Zircon SHRIMP II U-Pb dating indicates that a Late Cretaceous gabbro diorite pluton formed at 90.3 ± 1.4 Ma, and a Late Jurassic diorite porphyry pluton crystallized at 157.9 ± 2.3 Ma. The former has positive zircon εHf(t) values of 8.03–10.86 and whole-rock εNd(t) values of 4.11–4.37, with δ18Ozircon values of 5.84‰–6.70‰, slightly above the values of mantle zircon (5.3‰ ± 0.6‰). The latter also has positive zircon εHf(t) (10.29–12.77) and whole-rock εNd(t) (5.54–5.78) values, with normal mantle-like zircon δ18O values of 5.19‰ to 6.00‰. All samples of both plutons display arc-type signatures characterized by enrichment in light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti and P). Based on multiple isotopic (Sr–Nd–Hf–O) and whole-rock geochemical characteristics, we conclude that the Late Cretaceous gabbro diorite was derived from partial melting of depleted mantle that had been metasomatized by subducted-sediment-derived melts (~5 vol%). The Late Jurassic diorite porphyry is inferred to be a product of fractional crystallization of primary andesitic magmas with limited contents of Fe-Ti oxide and amphibole, and minor plagioclase accumulation. Pelagic sediments and enriched materials were a negligible component of the precursor magmas of the diorite porphyry, relative to the gabbro diorite pluton. Considering the temporospatial distribution of igneous rocks over the whole Gangdese magmatic belt, we suggest that Late Jurassic magmatism was related to normal angle subduction of the Yarlung Tsangpo Neo-Tethyan oceanic lithosphere, and early Late Cretaceous magmatism was triggered by rollback of the Neo-Tethyan oceanic slab.

Rock alteration at the post-Variscan nonconformity: implications for Carboniferous–Permian surface weathering versus burial diagenesis and paleoclimate evaluation

Abstract. A nonconformity refers to a hiatal surface located between metamorphic or igneous rocks and overlying sedimentary or volcanic rocks. These surfaces are key features with respect to understanding the relations among climate, lithosphere and tectonic movements during ancient times. In this study, the petrological, mineralogical and geochemical characteristics of Variscan basement rock as well as its overlying Permian volcano-sedimentary succession from a drill core in the Sprendlinger Horst, Germany, are analyzed by means of polarization microscopy, and environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and inductively coupled plasma mass spectrometry analyses. In the gabbroic diorite of the basement, the intensity of micro- and macro-fractures increases towards the top, indicating an intense physical weathering. The overlying Permian volcanic rock is a basaltic andesite that shows less intense physical weathering compared with the gabbroic diorite. In both segments, secondary minerals are dominated by illite and a mixed-layer phase of illite and smectite (I–S). The corrected chemical index of alteration (CIA) and the plagioclase index of alteration (PIA) indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ values for both basaltic andesite and gabbroic diorite indicate an abnormal enrichment of K, Rb and Cs that cannot be observed in the overlying Permian sedimentary rocks. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. The overall order of element depletion in both basaltic andesite and gabbroic diorite during the weathering process is as follows: large-ion lithophile elements (LILEs) > rare earth elements (REEs) > high-field-strength elements (HFSEs). Concerning the REEs, heavy rare earth elements (HREEs) are less depleted than light rare earth elements (LREEs). Our study shows that features of supergene physical and chemical paleo-weathering are well conserved at the post-Variscan nonconformity despite hypogene alteration. Both can be distinguished by characteristic minerals and geochemical indices. Based on these results, a new workflow to eliminate distractions for paleoclimate evaluation and evolution is developed.

Early Neoproterozoic continuous oceanic subduction along the northern margin of the Tarim Block: Insights from ca. 910–870 Ma arc-related magmatism in the Aksu area, NW China

The Tarim Block in NW China records the tectonic transition from compression to extension during the Neoproterozoic, and hence is regarded as a component of the Rodinia supercontinent. To unravel the role of the Tarim Block in the Rodinia assembly during the early Neoproterozoic, new geochronological and geochemical analyses were conducted on the gabbroic diorite, plagiogranite and andesite exposed in the Aksu area, northwestern Tarim. Zircon U-Pb dating indicates that these magmatic rocks were formed at ca. 900–870 Ma. Geochemically, these magmatic assemblages have typical features of arc-type rocks, showing enrichment of LILEs and depletion of HFSEs (such as Nb, Ta and Ti). They all have positive zircon εHf(t) values (+6.8 to +12.7), positive εNd(t) values (+3.6 to +6.0), and low and constant (87Sr/86Sr)i ratios (0.70252 to 0.70485). The gabbroic diorites display geochemical features of boninitic rocks such as high SiO2 (53.4–56.3 wt%), MgO (5.79–9.15 wt%), low TiO2 (0.29–0.41 wt%) contents, indicating that they were derived by partial melting of residual mantle. The plagiogranites are characterized by high SiO2 (74.7–79.6 wt%), Na2O (3.87–4.74 wt%), moderate Al2O3 (11.5–13.2 wt%), low K2O (0.15–0.22 wt%) contents, with very low TiO2 (0.08–0.17 wt%) and total REEs (6.3–19.0 ppm) contents similar to oceanic plagiogranite, indicating a hydrous low-pressure melting of mafic protolith. The positive εNd(t) and zircon εHf(t) values and low (87Sr/86Sr)i ratios of the andesitic rocks in the Aksu area suggest a depleted mantle origin. The analysis of trace elements (e.g. Th, Zr, Nb) of these intrusive and volcanic rocks in the Aksu area indicates that their magma sources had been metasomatized by slab-derived fluids. Combined with trace element features, we infer that these early Neoproterozoic magmatic rocks in the Aksu area developed in an oceanic subduction setting with arc-back-arc system probably analogous to the Izu-Bonin (IB) subduction system in the western Pacific. These findings lead us to propose that continuous oceanic subduction beneath the northern margin of the Tarim Block occurred during ca. 910–870 Ma, corresponding to the Neoproterozoic circum-Rodinia subduction-accretion system, which provides new evidence for the accretionary process of the Tarim Block to Rodinia during the early Neoproterozoic.

Proto-Tethys ophiolitic mélange in SW Yunnan: Constraints from zircon U-Pb geochronology and geochemistry

An early Paleozoic Proto-Tethys ocean in western Yunnan has long been postulated although no robust geological evidence has been identified. Here we investigated the recently-identified Mayidui and Wanhe ophiolitic mélanges in SW Yunnan, which occurs in a N–S trending belt east of the late Paleozoic Changning–Menglian suture zone. The ophiolites consist mainly of meta-basalts (amphibole schists), meta- (cumulate) gabbros and gabbroic diorites, and meta- chert-shale, representing ancient oceanic crust and pelagic and hemipelagic sediments, respectively. Six samples of gabbros and gabbroic diorites from 3 profiles (Mayidui, Kongjiao and Yinchanghe) yielded zircon U-Pb ages between 462 ± 6 Ma and 447 ± 9 Ma, constraining the formation of the Mayidui and Wanhe ophiolites to Middle Ordovician. Gabbros from the Mayidui and Kongjiao profiles share similar geochemical characteristics with affinities to tholeiitic series, and are characterized by depleted to slightly enriched LREEs relative to HREEs with (La/Sm)N = 0.69–1.87, (La/Yb)N = 0.66–4.72). These, along with their predominantly positive whole-rock εNd(t) and zircon εHf(t) values, indicate a MORB-like magma source. By contrast, the meta-mafic rocks from the Yinchanghe profile show significantly enriched LREEs ((La/Sm)N = 0.97–3.33, (La/Yb)N = 1.19–14.93), as well as positive whole-rock εNd(t) and positive to negative zircon εHf(t) values, indicating an E-MORB-type mantle source. These geochemical features are consistent with an intra-oceanic setting for the formation of the Mayidui–Wanhe ophiolites. Our data, integrated with available geological evidence, provide robust constraints on the timing and nature of the Mayidui–Wanhe ophiolitic mélange, and suggest that the ophiolites represent remnants of the Proto-Tethys Ocean, which opened through separation of the Indochina and Simao blocks from the northern margin of Gondwana before the Early Cambrian, and evolved through to the Silurian.

Petrogenesis and oxygen fugacity of the Xintian complex associated with the Naozhi-Xintian porphyry–epithermal system in the Yanbian area, NE China

• The Xintian complex is associated with a telescoped porphyry-epithermal system. • Influxes of mafic melts into felsic melts result in the emplacement of this complex. • High oxidation state and magmatic water are conducive to the Cu–Au mineralization. The Naozhi polymetallic veins classified into the intermediate-sulfidation (IS) type and the 2 km northwest Xintian porphyry Cu–Mo ore occurrence in the Yanbian area, Northeast China, constitute a telescoped porphyry-epithermal system. IS veins are surrounded by Early Jurassic granodiorite stocks (189 ± 1 Ma; zircon U-Pb ) and Early Cretaceous andesite lava (128 – 130 Ma; zircon U-Pb ). They are partially covered by younger pyroclastic rocks dated at 108 ± 2 Ma (Zircon U-Pb ). The Xintian occurrence is connected in space and time with an intrusive complex including five sequential phases from gabbro diorite and granodiorite stocks through diorite porphyry dykes to granodiorite porphyry and granite porphyry stocks, with indistinguishable ages within errors (128 ± 1 Ma to 126 ± 1 Ma zircon 206 Pb/ 238 U by laser ablation inductively coupled plasma mass spectrometry). The broad compositional variations (eg., 52.24 – 69.23 wt% SiO 2 ) of the complex and the igneous mafic microgranular enclaves (MMEs) hosted by granodiorite stocks are likely responses to inputs of hot mafic melts into a shallow felsic magma chamber. Based on the trace element contents of zircons, the redox conditions during the crystallization of zircon minerals in equilibrium with the melts are evaluated by calculating the FMQ values. The early mafic phase (gabbro diorite; FMQ 4.60 ± 1.33) is more oxidized than later phases (granodiorite, diorite porphyry, granodiorite porphyry, and granite porphyry) with FMQ values of 1.70 ± 1.80 to 2.28 ± 2.36. The highly oxidizing conditions of associated magmas, as well as high magmatic water contents (>4%) because of ubiquitous biotite and amphibole in this complex, are conducive to escape of primary volatiles from shallow, fertile porphyry magmas, with consequent ore deposition in structurally optimal positions at Naozhi-Xintian.

Mafic to intermediate composition intrusions from the Kahak area, central Urumieh-Dokhtar arc of Iran: transition from Eocene to Miocene intra-arc extensional magmatism

Several gabbro, gabbroic diorite, and diorite intrusions were emplaced in the Neotethyan Urumieh-Dokhtar magmatic arc in the Kahak area of the Zagros Orogen. The plutonic rocks intruded Cenozoic volcanic and pyroclastic succession. U-Pb dating via LA-ICP-MS of zircon yields early to middle Eocene ages of ~53 to 38 Ma for the diorite and gabbroic diorite in Veshnaveh and Naragh areas and Miocene ages of 23 to 20 Ma for gabbroic rocks in the Kerogan and Fordou areas. The Eocene magmatic phase occurred quite possibly as three continuous pulses. The Eocene and Miocene intrusions have negative Nb-Ta anomalies like those of subduction-related magmas. The Pb isotopic compositions show the involvement of subducted sediment in the source of the studied samples. The Eocene intrusions are tholeiitic and enriched in incompatible-trace-elements (especially the HFSE) compared to the Miocene intrusions which are calc-alkaline and have lower concentrations of most incompatible trace elements. Sr-Nd isotopic compositions of the Eocene rocks lie close to the mantle array and near Bulk Silicate Earth, but the Miocene suite is shifted to higher (87Sr/86Sr)i. The higher (87Sr/86Sr)i, along with larger Ba, K, Pb, and Sr anomalies, in the Miocene suite imply more slab fluid input and/or crustal contamination. Apparently, the Eocene mafic magmas formed in slightly metasomatized sub-continental lithospheric mantle (SCLM), whereas the Miocene suite was derived from highly metasomatized, deeper levels of the SCLM. We suggest that slab roll back occurred between the Eocene and Miocene and transferred the location of mantle partial melting to greater depths.

Integrated Laser-Induced Breakdown Spectroscopy (LIBS) and Multivariate Wavelet Tessellation: A New, Rapid Approach for Lithogeochemical Analysis and Interpretation

This paper demonstrates a novel approach that uses wavelet tessellation in rapid analysis of raw geochemical data produced by laser-induced breakdown spectroscopy (LIBS) to produce pseudologs that are representative of stratigraphy. Single-line LIBS spectral data for seven major rock-forming elements (Al, Ca, Fe, Mg, Si, Na and K) were collected from a synthetic 22-sample rock-block comprising two distinct lithological groups based on mineralogy, chemistry and texture: plutonic rocks and marble. Seven sublithologies are identified within the rock-block from traditional laboratory whole-rock geochemical analysis: marble, Mg-marble, granite, quartz monzonite, foidolite, granodiorite and gabbroic diorite. Two-domain clustering (k = 2) on raw spectral LIBS data combined with wavelet tessellation was applied to generate a simplified lithological stratigraphy of marble and plutonic rocks and generate a pseudolog identical to the rock-block stratigraphy. A pseudolog generated from seven-domain clustering (k = 7) and wavelet tessellation successfully discriminated most sublithologies within the rock-block slabs, especially marble slabs. Small-scale units were identified within the more mineralogically and geochemically complex plutonic slabs. The spatial resolution of the LIBS analysis, with a measurement spacing of ~0.35 mm, allowed for assessment of individual mineral compositions and rock textures, and small-scale units within the plutonic rocks can be correlated to specific coarse-grained minerals or mineralogical associations. The application of the wavelet tessellation method to raw LIBS geochemical data offers the possibility of rapid and objective lithogeochemical analysis and interpretations which can predate further analysis (quantitative) and supplement geological logging.

Early Permian continental arc magmatism in the Zhaojinggou area of the northern North China Craton: Implications for crust-mantle interactions during southward Paleo-Asian plate subduction

Early Permian magmatism in the Zhaojinggou area of Inner Mongolia, northern China produced intrusive gabbroic diorite and granites including biotite granite, K-feldspar granite and fine-grained granite. We present zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions of these rocks to resolve complex crust–mantle processes beneath the northern margin of the North China Craton (NCC). Zircon U–Pb dating reveals that the rocks were emplaced in the Early Permian, with gabbroic diorite (291.1 ± 1.8 Ma) forming ~10 Myr earlier than the granites (biotite granite, 280.0 ± 1.6 Ma; K-feldspar granite, 279.7 ± 2.1 Ma; fine-grained granite, 282.2 ± 2.9 Ma). Geochemical data indicate that the rocks have calc-alkaline to high-K calc-alkaline and metaluminous to peraluminous affinities, with enrichment in light rare-earth elements (LREE) and large-ion lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), typical of continental arc magmatic rocks. The gabbroic diorite is characterized by low–moderate Mg# values (38–59, mean 50), moderate average Cr (45.0 ppm) and Ni (36.5 ppm) contents, and moderately enriched Sr–Nd–Hf isotopic compositions with (87Sr/86Sr)i values of 0.704719–0.705497; εNd(t) values from −7.8 to −7.0; and εHf(t) values from −11.2 to −6.7. These features, combined with highly variable Rb/Y, Ba/La, Th/Yb, Th/Ce and Th/Sm ratios and Ba contents, indicate that the gabbroic diorite was derived from subduction-related enriched sub-continental lithospheric mantle (SCLM), previously metasomatized by slab fluids and ~5% sediment melts. Weakly negative to strongly positive Eu anomalies among gabbroic diorite samples indicate no residual plagioclase accumulation or fractionation during magma evolution. The granites have similar ages and enriched isotopic features (87Sr/86Sri = 0.703611–0.707427; εNd(t) = −11.4 to −9.2; εHf(t) = −12.8 to −7.2) with old Nd and Hf model ages of 1980–1801 Ma and 1730–2088 Ma, respectively, indicating that they originated through partial melting of ancient mafic lower crust. Both biotite granite and K-feldspar granite exhibit weakly negative to highly positive Eu anomalies and relatively low Zr saturation temperatures of 618–648 °C. In contrast, the fine-grained granite has negative Eu anomalies and relatively high Zr saturation temperatures (692–709 °C). These features imply that the magma source of the fine-grained granite (with plagioclase accumulation) was deeper than that of the biotite granite and K-feldspar granites (without plagioclase accumulation). Different mantle and crustal sources involved in the generation of continental arc magmatism in the Zhaojinggou area have implications for extensive and complex crust–mantle interactions beneath the northern margin of the NCC related to southward subduction of the Paleo-Asian oceanic crust during the Early Permian.

Paleoproterozoic Orosirian tectono-thermal events in the Nangrim Massif, North Korea: Cratonic and supercontinental connection

The Paleoproterozoic Orosirian period of ca. 2.05–1.8 Ga saw unique magmatic manifestations of Andean-type arc to Caledonian-style post-collisional batholiths, anorthosite–mangerite–charnockite– granite suites and mafic dyke swarms. Chronicling their timeline in North Korea presents a pivotal task in the palinspatic reconstruction of the Korean Peninsula relative to the North China Craton (NCC) and the supercontinent Columbia. Here we document episodic late Orosirian tectono-thermal events from the northern Nangrim Massif, spanning from ca. 1871–1851 Ma tonalitic intrusions through ca. 1856 Ma granites and ca. 1839 Ma gabbroic diorites to ca. 1830 Ma mafic dykes and ca. 1828 Ma metamorphism. Meta-gabbroic/doleritic rocks range in SiO2 from 46.6 to 57.8 % and in MgO from 6.46 to 11.5 %, with moderately enriched light rare earth elements (LaN/YbN = 3.55–12.3) and depleted high field strength elements. Together with their variable zircon εHf(t) values from −10.0 to +5.2, these rocks tend to call for a heterogeneous lithospheric mantle that experienced prior melt depletion and subsequent fluid refertilization. Pyroxene-bearing tonalite–granite suite spans in SiO2 from 59.0 to 76.4 % and exhibits transitional character from magnesian and alkalic to ferroan and calc-alkalic. These elemental features and zircon εHf(t) values of −7.9 to −1.0 are consistent with partial melting of newly- underplated mafic middle crustal protoliths. By combining these newly identified tectono-thermal events with massif-scale coeval syenite-granite suites as well as anatexis-induced metamorphic pulses, we updated the profile of Orosirian tectono-thermal episodes in the Nangrim Massif, which present successive snapshots into post-collisional geodynamic processes from orogenic collapse to lithospheric delamination. This renewed profile not only provides convincing evidence for demarcating the influence scope of the giant Paleoproterozoic Jiao-Liao-Ji-Korean (JLJK) orogeny in the Korean Peninsula, but also offers key reference for applying analogous Phanerozoic accretionary to collisional orogenic processes to their ancient tectono-magmatic expressions. The JLJK case of Orosirian orogeny exemplifies the importance of Phanerozoic-style Wilson orogenic cycle in the amalgamation of the Sino-Korean craton and its incorporation into the Paleoproterozoic supercontinent Nuna–Columbia.

Petrography and geochemistry of intrusive magmas in Varmaqan – Sardare ghobadi in the west of Iran

The study area is a quadrilateral of 155 km2 between eastern longitude 47˚ and 40 ′ to 47˚ and 52 ′ and northern latitudes 35˚ and 00 ′ to 35˚ and 04 ′ that is located in west of Iran, north of Sonqor city and between Varmaqan and Sardare Ghobadi villages of Kermanshah province. In this range, the intrusive rocks are alkaline granite, granite, granodiorite, tonalite, quartz alkaline syenite, quartz monzonite, quartz monzodiorite, quartz diorite, alkaline syenite, monzonite, diorite, gabbro diorite, gabbro, and olivine gabbro as they were injected in the iron ores of cretaceous which has resulted in contact metamorphism and created hornfels at the site of contact. After comprehensive sampling of all required igneous rocks and according to the thesis objectives, thin sections were prepared and after petrography and some samples were selected for geochemical experiments. XRF analysis, ICP and alkaline fusion were performed on some samples. According to geochemical and petrological studies, the magmas forming these intrusive igneous rocks are from one region and because of magmatic differentiation or fractional crystallization, they from basaltic to acidic terms. Samples of this quadrilateral have a meta-alumina nature and granitoids are in the range of arc islands granites, continental arc granitoids and continental collision granitoids. The mineralogical and chemical composition of the acidic rocks in the area show that the granites in this study are type I.

Study of the petrography and tectonic settings of sills in Lavasanat district, Tehran (north of Iran)

The study area is located in Lavasanat District in the northeast of Tehran in Central Alborz zone. The outcrops are mainly linked to Karaj Formation, which belongs to the upper Eocene to Oligocene periods. In the study area, there are various plutonic rocks that are identified in the form of numerous dike and sill on the ground. These sills are injected in between sedimentary layers. The rocks forming the sills include the spectrum of gabbro, gabbro diorite, diorite, monzonite, and syenite. In some areas, these rocks have undergone alterations and have traces of the saussuritization and chloritization phenomena. There are also two generations of amorphous. The first generation is fully chloritized due to alteration while the second generation is unaltered. Unlike many diorites and monzonites, which typically have hornblendes, the neutral rocks such as diorite and monzonite lack hornblendes. Hence, their magmas were dry and dehydrated. Based on the geochemical studies conducted on 17 samples (15 sill samples and 2 host rock samples) and the diagrams of the tectonic settings of rocks, the study sills are in the WIN (within plate). However, two samples of the host rock are within the range of the active continental margins.

The role of hydrous mantle-derived magmas in the generation of Late Cretaceous granitoids in the Gangdese batholith: insights from the Shanba and Zongga plutons in the southern Lhasa subterrane, Tibet

We present an integrated study involving detailed field investigation, petrography, whole-rock geochemistry, Sr–Nd isotopic compositions, and zircon U–Pb–Hf isotopic compositions of Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons of the southern Lhasa subterrane, Tibet. We use the obtained data to assess the genetic relationship between the granitoids and gabbroic diorites and to better constrain the origin of Late Cretaceous granitoids in the Gangdese batholith. Both the Shanba and Zongga plutons contain Late Cretaceous gabbroic diorites with zircon U–Pb ages of 91–90 Ma, but only the Zongga pluton includes coeval Late Cretaceous (88–83 Ma) granitoids (tonalites and granodiorites) that coexist with the gabbroic diorites. The Shanba gabbroic diorites consist mainly of clinopyroxene, biotite, and plagioclase, whereas the Zongga gabbroic diorites are composed predominantly of amphibole and plagioclase. The Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons are enriched in large-ion lithophile elements and light rare earth elements (REEs), and are depleted in high-field-strength elements and heavy REEs, showing an arc-type geochemical signature. The Zongga gabbroic diorites have lower K2O and light REE contents and Th/Y ratios, as well as higher Ba/La, Ba/Nb, and Rb/Nb ratios, compared with the Shanba gabbroic diorites. The Zongga gabbroic diorites and granitoids have similar Sr–Nd–Hf isotopic compositions [ISr = 0.7035–0.7042, eNd(t) = 5.0–6.4, eHf(t) = 9.6–13.8], with more radiogenic Nd isotopic compositions than those of the Shanba gabbroic diorites [ISr = 0.7037–0.7040, eNd(t) = 3.1–4.1, eHf(t) = 10.9–14.1]. These geochemical and isotopic data indicate that the Zongga granitoids are genetically related to their spatially and temporally associated Zongga gabbroic diorites, whose mantle source was more hydrous than that of the Shanba gabbroic diorites. On the basis of the petrological, geochemical, and isotopic data, we propose that the Shanba and Zongga gabbroic diorites were generated by partial melting of a depleted mantle wedge that was metasomatized by slab sediment melts and by slab dehydration fluids, respectively. The Zongga granitoids were produced by dehydration melting of juvenile lower crust compositionally similar to the Zongga gabbroic diorites, and the parental magma of these gabbroic diorites was also involved in their generation. We conclude that Late Cretaceous underplating of mantle-derived magmas with differing water contents formed juvenile lower crust in the southern Lhasa subterrane; however, only the more hydrous juvenile lower crust was able to produce voluminous granitoids through dehydration melting. Hydrous mantle-derived magmas, therefore, played a key role in the generation of Late Cretaceous granitoids in the Gangdese batholith and contributed not only to crustal growth but also to crustal reworking in the southern Lhasa subterrane.

U–Pb zircon ages and petrogenesis of the Late Cretaceous I-type granitoids in arc setting, Eastern Pontides, NE Turkey

The Eastern Pontides Orogenic Belt (EPOB) as a well-preserved ancient magmatic arc situated in the eastern part of the Sakarya Zone and has key importance to allow for proper reconstruction of the geodynamic evolution of the region. The region accepted as important part of Alpine–Himalayan Belt comprises of many granitoids of the Late Cretaceous age. This paper presents new mineral chemistry, bulk-rock petrochemistry, U-Pb geochronology and radiogenic and stable isotope geochemistry data from the Arpaköy and Oyman granitoids in the north part of the EPOB to interpretation of the igneous activity and geodynamic evolution of the region in Late Cretaceous era. Emplacement ages of 86.82 ± 0.58 Ma for the Oyman granitoid and 82.0 ± 2.3 Ma for the Arpaköy granitoid according to laser ablation ICP-MS U–Pb zircon dating. The Arpaköy granitoid are gabbro diorite, diorite and granite, whereas the Oyman granitoid are tonalite and granite in composition. They show metaluminous-peraluminous transitional (ASI = 0.79–1.20) low to medium-K calc-alkaline features. The rock samples from granitoids are characterized by enrichment in large ion lithophile elements (LILE) and high field strength elements (HFSE) depletion with concave-upward shapes (LaN/LuN = 1.88 to 6.04). The crystallization temperatures are 648 °C–913 °C and the pressure values are 0.2–3.0 kbar, which indicates emplacement depth of the studied granitoids varying from ~1 to 9 km correspond to shallow crustal depths. They show considerable variations in εNd(i) (−4.02 to 1.50) and 87Sr/86Sr(i) (0.70478–0.70689). The 206Pb/204Pb ratios are between 17.97 and 18.65, and δ18O are varying from +5.4 to +9.3‰. The parental magma of the Arpaköy and Oyman granitoids originated from a melt of the enriched lithospheric mantle and the fractional crystallization and lesser assimilation processes has important role on the evolution of magmas in the shallow crustal levels.

Neoproterozoic Trench‐arc System in the Western Segment of Jiangnan Orogenic Belt, South China

AbstractThe Jiangnan orogenic belt is a key to understanding of the Neoproterozoic tectonic evolution of the South China Block. We investigate the mafic‐ultramafic suites of lherzolite, pyroxenite, gabbro, pillow basalt and gabbroic diorite as well as red jasper interbedded with marine marbles that are mainly exposed as fault‐trapped blocks in the Yuanbaoshan and Longsheng domains of the western Jiangnan belt. The post‐collisional granite plutons that intruded the ultramafic‐mafic rocks are developed well. Zircons in the gabbro yield crystallization ages of 867±10 Ma, 863±8 Ma, 869±9 Ma and 855±5 Ma whereas those from the granites show ages of 823±5 Ma, 831±5 Ma, 824±5 Ma and 833±6 Ma. The Neoproterozoic serpentinited ultramafic samples display minor REE enriched pattern with depletion of Rb, Ba, Nb, Ta and Ti, similar to those of SSZ type ophiolite. The coeval gabbro shows tholeiitic features and is characterized by negative Ba, Nb, Ta, Zr, Hf and Sr anomalies and LREE enriched patterns, with a minor negative Eu anomaly. Some zircon grains from the Longshen gabbro yield Neoarchean‐Paleoproterozoic ages (2859–2262 Ma), suggesting its continental arc setting. Geochemical signature of the mafic‐ultramafic rocks is consistent with subduction related setting. The pyroxene‐bearing diorite exhibits a distinctive arc affinity. The zircons from the gabbro show positive ∊Hf(t) values ranging from 3.9 to 13.8. The granitoids are typical S–type granites with high ACNK values (1.15–1.40) and negative ∊Hf(t) values (−15.1 to −3.2), and are classified as collision–related granites. Combined with the occurrences of mafic‐ultramafic rocks, siliceous marble and red jasper mixed with basalt, our new results suggest the presence of a Tonian (863–869 Ma) SSZ ophiolite system and continental arc‐type magmatism in the western Jiangnan orogen.

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes

Eocene arc magmatism and related Cu-Au (Mo) mineralization in the Shangalon-Kyungalon district, Wuntho-Popa Arc, northern Myanmar

The Shangalon-Kyungalon Cu-Au (Mo) ore district is located on the south-eastern margin of the Kanzachaung batholith in the Wuntho-Popa Arc, Myanmar. Porphyry and epithermal Cu-Au mineralization are associated with the Late-Eocene magmatic rocks. In this study, zircon U-Pb dating of gabbroic diorite, diorite porphyry, tonalite porphyry, and granodiorite yielded ages of 38.0 ± 0.35 Ma, 37.8 ± 0.35 Ma, 37.4 ± 0.27 Ma, and 36.9 ± 0.26 Ma, respectively. These rocks show sub-alkaline characteristics, LREE-enriched patterns with weakly positive/negative Eu anomalies, enrichment of LILEs and depletion of HFSEs. They have obviously higher initial Sr ratios and lower εNd(t) and εHf(t) values (Isr = 0.7066; εNd(t) = −0.5 to −4.8; εHf(t) = 0.7–9.7) than the mid-Cretaceous Kanzachaung batholith (Isr = 0.7046; εNd(t) = 2.4–4.5; εHf(t) = 10.9–15.1), indicating crustal materials were most likely added to the arc magma source. In particular, the gabbroic diorite has higher initial Sr, more enriched Nd-Hf isotopic compositions (Isr = 0.7081, εNd(t) = −4.6 to −4.8, εHf(t) = 0.7–2.3) and lower SiO2 content relative to the other magmatic rocks studied (Isr = 0.7068–0.7059; εNd(t) = −0.5 to −2.8, εHf(t) = 5.0–9.7). The results suggest that the gabbroic diorite was derived from melting of a mantle wedge, metasomatized by subducted oceanic sediment. Underplating on juvenile lower crust followed, accompanied by MASH processes, which led to increased εNd(t) (and SiO2 content) and decreased initial Sr ratios in later formed rocks. Moreover, the adakite-like features (Sr/Y = 19.3–53.4, Y = 7.54–18.8,) suggest fractional crystallization of hornblende in a hydrous magma, potentially leading to formation of the Shangalon-Kyungalon porphyry-epithermal Cu-Au system. In summary, the Late-Eocene magmas in Shangalon-Kyungalon formed from a mantle source metasomatized by subducted sediments during Neo-Tethyan subduction beneath Western Burma terrane, with the involvement of juvenile crustal materials.

GEOCHEMISTRY AND PETROGENESIS OF SILLS IN LAVASANAT REGION, TEHRAN, IRAN

The study area is located in Lavasanat region in the east of Tehran Province in the Central Alborz zone. The outcrops in this region are mainly associated with Karaj formation and belong to the upper Eocene to Oligocene periods. These outcrops consist of two intrusions: one in the north and the other in the northeast of Lavasanat. These outcrops are scattered over an area of approximately 337km2. In the study region, there are various intrusive igneous rocks, while numerous intrusive igneous rocks are scattered in the form of sills and dikes. These rocks include a range of rocks from diorite gabbro to diorite, monzonite, and syenite. The weathered colors of these rocks are black, brown and gray. The outcrops of these sills are mainly present in the middle-northern part of the quadrangle geological map of the east of Tehran. In different diagrams of the magma series, the study rocks are classified as alkaline, calc-alkaline, and shoshonite, which may indicate magma contamination. An analysis of the variations of the classical elements and trace elements, the spider plots, and interpretations of these plots confirms the relationship of these rocks with the subduction zone and continental arc. To find the tectonic position of the sills in the study area based on the geochemical diagrams, we selected samples from the within-plate (WIP) and arc zones.

Apatite (U–Th)/He thermochronometry of iron deposits in the central Ningwu volcanic basin, eastern China: Implications for exhumation and preservation

Apatite (U–Th)/He thermochronometry (AHe) was used to constrain the exhumation history of the Taocun and Washan iron oxide deposits, located in the central Ningwu volcanic basin of the Lower Yangtze River Belt (LYRB) in eastern China. The porphyritic gabbro–diorite host rock yields zircon U–Pb ages of ~130 Ma, which are different to the apatite (U–Th)/He ages from this study. The Taocun iron deposit was emplaced below the apatite helium partial retention zone (PRZ) and its AHe ages (17.8 ± 1.0 to 19.5 ± 1.2 Ma) reveal a rapid exhumation-induced cooling event that began at ~23 Ma. The Washan iron deposit, which is hosted within a cryptoexplosion breccia, was probably emplaced in the apatite helium PRZ. It yields two groups of AHe ages: an older group (58.5 ± 3.3 and 57.8 ± 3.5 Ma) and a younger group (27.0 and 22.8 Ma). The youngest AHe age from the Washan iron deposit (22.8 Ma) records the same rapid exhumation-induced cooling event that is recorded in the Taocun deposit AHe ages. This event was related to structural inversion in eastern China that began during the early Miocene (~23 Ma). This inversion marked the end of widespread extension that started in the Early Cretaceous, and it was accompanied by the formation of structures related to Neogene and Quaternary regional compression. Given the relatively recent timing of the exhumation, the overall amount of exhumation of the central Ningwu Basin is relatively minor. We integrated our results with the porphyrite iron deposit model and existing information on the Xuejin detachment fault. It seems that iron deposits of the deepest (Gushan) sub-type of the porphyrite iron deposit model may still be preserved at depth in the central Ningwu Basin.

The Atud gabbro–diorite complex: glimpse of the Cryogenian mixing, assimilation, storage and homogenization zone beneath the Eastern Desert of Egypt

We analysed gabbroic and dioritic rocks from the Atud igneous complex in the Eastern Desert of Egypt to understand better the formation of juvenile continental crust of the Arabian–Nubian Shield. Our results show that the rocks are the same age (U–Pb zircon ages of 694.5 ± 2.1 Ma for two diorites and 695.3 ± 3.4 Ma for one gabbronorite). These are partial melts of the mantle and related fractionates (εNd 690 = +4.2 to +7.3, 87 Sr/ 86 Sr i = 0.70246–0.70268, zircon δ 18 O ∼ +5‰). Trace element patterns indicate that Atud magmas formed above a subduction zone as part of a large and long-lived ( c . 60 myr) convergent margin. Atud complex igneous rocks belong to a larger metagabbro–epidiorite–diorite complex that formed as a deep crustal mush into which new pulses of mafic magma were periodically emplaced, incorporated and evolved. The petrological evolution can be explained by fractional crystallization of mafic magma plus variable plagioclase accumulation in a mid- to lower crustal MASH zone. The Atud igneous complex shows that mantle partial melting and fractional crystallization and plagioclase accumulation were important for Cryogenian crust formation in this part of the Arabian–Nubian Shield. Supplementary material: Analytical methods and data, calculated equilibrium mineral temperatures, results of petrogenetic modeling, and cathodluminesence images of zircons can be found at https://doi.org/10.6084/m9.figshare.c.4958822

Opening and closure history of the Mudanjiang Ocean in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from early Mesozoic intrusive rocks

The tectonic evolution of the ancient Mudanjiang Ocean within the Central Asian Orogenic Belt (CAOB), is strongly debated. The ocean played an important role in the amalgamation of the Songnen and Jiamusi massifs; however, the timings of its opening and closure have remained ambiguous until now. In this study, we analyzed early Mesozoic intrusive rocks from the eastern Songnen and western Jiamusi massifs in the eastern CAOB. The new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data are used to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon UPb dating indicates that early Mesozoic magmatism in the eastern Songnen Massif occurred in three stages: Early to Middle Triassic (ca. 250 Ma), Late Triassic (ca. 211 Ma), and Early Jurassic (ca. 190 Ma). The Triassic intrusive rocks typically consist of bimodal rock suites, which include gabbros, hornblende gabbros, and granitoids. The compositional information indicates an extensional environment that was probably related to the final closure of the Paleo-Asian Ocean. We integrated the results with observations from Triassic A-type granitoids and coeval sedimentary formations in the eastern Songnen Massif, as well as depositional ages of metasedimentary rocks from Heilongjiang Complex. We conclude that the opening of the Mudanjiang Ocean took place in the Early to Middle Triassic. The Early Jurassic intrusive rocks are bimodal and include olivine gabbros, hornblendites, hornblende gabbros, gabbro diorites, and granitoids. The bimodal rock suite indicates a back-arc style extensional environment. This setting formed in relation to westward subduction of the Paleo-Pacific plate beneath the Eurasia during the Early Jurassic. Following subduction, the closure of the Mudanjiang Ocean and subsequent amalgamation of the Songnen and Jiamusi massifs happened during the late Early Jurassic to Middle Jurassic. This sequence of events is further supported by ages of metamorphism and deformation acquired from the Heilongjiang Complex. Based on these observations, we conclude that the Mudanjiang Ocean existed between the Middle Triassic and Early Jurassic, making it rather short-lived.