Post-collisional Extensional Environment Research Articles

Geochronology and geochemistry of Late Devonian-Carboniferous igneous rocks in the Songnen-Zhangguangcai Range Massif, NE China: Constraints on the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

This paper presents new zircon U–Pb ages and Hf isotopic data, and whole–rock geochemical data for Late Devonian–Carboniferous igneous rocks from the Songnen–Zhangguangcai Range Massif (SRM), NE China, to constrain the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt and the nature of the crust beneath the SRM. LA–ICP–MS zircon U–Pb dating reveals two stages of magmatism in the Late Devonian (366Ma) and Carboniferous (325–313Ma). The Late Devonian magmatic event produced alkali feldspar granites with high SiO2 and total alkali contents, as well as pronounced negative Eu anomalies, similar to A-type granites. Based on these features, their high zircon εHf(t) values (+9.6 to +13.0), and the early Paleozoic tectonic evolution of the eastern SRM, we conclude that the primary magma for the Late Devonian granites was generated by partial melting of juvenile lower crust in a post-collisional extensional environment. The Carboniferous magmatic event produced a suite of intermediate–felsic igneous rocks. The intermediate rocks, including basaltic andesite, andesite, and quartz monzodiorites, have low SiO2 contents, high Mg#, high Cr, Co, and Ni contents, and display arc-like trace element characteristics. Based on these observations and their high zircon εHf(t) values (+5.8 to +10.7), we suggest their primary magma was derived from partial melting of depleted mantle, metasomatized by subduction-related fluids. The felsic rhyolites and monzogranites have high SiO2 contents, low Mg#, extremely low Cr, Co, and Ni contents, and zircon εHf(t) values of +10.5 to +13.7 and +4.2 to +6.9, respectively. These results indicate that the primary magma for the felsic rocks was derived from partial melting of heterogeneous juvenile crust. The Carboniferous igneous rock assemblages, together with coeval carbonate sedimentary formations, record an intra-plate extensional setting. The zircon εHf(t) data of Paleozoic igneous rocks from the eastern SRM define a temporal trend that is consistent with tectonic evolution. The increase in εHf(t) values during the early late Paleozoic age mirrors changes in magma source, corresponding to a switch from subduction–collision to post-collision tectonic regimes in the SRM. The zircon Hf isotopic compositions of the studied Late Devonian–Carboniferous igneous rocks record crustal accretion in the eastern SRM during the Neoproterozoic–early Paleozoic. Reworking of these crustal materials occurred in a post-collisional extensional environment during the late Paleozoic.

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area, southern Great Xing’an Range, NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

ABSTRACTThis study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon εHf(t) values from +4.06 to +11.62 that yield two-stage model ages (TDM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A2-type affinities. These volcanics have εHf(t) and TDM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.

Petrogenesis and tectonic setting of the Middle Permian A‐type granites in Altay, northwestern China: Evidences from geochronological, geochemical, and Hf isotopic studies

The Jiangjunshan and Dakalasu alkali‐feldspar granites are located in the central part of the Chinese Altay orogen. In this paper, we present detailed geochemical, zircon U–Pb, and Hf isotopic data of these granites. The Jiangjunshan and Dakalasu alkali‐feldspar granites show a high content of SiO2 (72.05–73.27 and 69.55–71.04 wt%, respectively), total alkalis (Na2O + K2O = 8.41–8.71 and 7.24–8.66 wt%, respectively), and high‐field strength elements (Zr + Nb + Ce + Y = 400.3–482.9 and 156.7–339.3 ppm, respectively), as well as high Ga/Al ratios (10,000 × Ga/Al = 3.46–4.19 and 2.62–3.28, respectively) and depletion in Ba, Nb, Sr, and Ti, showing geochemical characteristics similar to those of A‐type granites. Zircon U–Pb dating of the Jiangjunshan and Dakalasu alkali‐feldspar granites yielded weighted mean dating 206Pb/238U ages of 268.3 ± 1.9 and 270.4 ± 1.9 Ma, respectively, indicating that these granites intruded during the Permian. The Jiangjunshan and Dakalasu alkali‐feldspar granites show highly variable zircon εHf(t) values ranging from −7.0 to +5.6, implying that these granites originated from a mixing of mantle‐derived magma with crustal materials. Our data on the Jiangjunshan and Dakalasu alkali‐feldspar granites, coupled with previous studies of Permian magmatism and metamorphism, suggest that the tectonic regime was in a postcollisional extensional environment in the Chinese Altay orogen during the Permian. Therefore, the change in stress from compression to extension and asthenospheric upwelling triggered by slab break‐off plays a significant role in the generation of Jiangjunshan and Dakalasu alkali‐feldspar granites.

Geochronology and geochemistry of early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China: Implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

This paper presents new zircon U–Pb, Hf isotope, and whole-rock major and trace element data for early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China, in order to constrain the early Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating indicates that early Paleozoic magmatic events within the northern Songnen–Zhangguangcai Range Massif (SZM) can be subdivided into four stages: Middle Cambrian (~505Ma), Late Cambrian (~490Ma), Early–Middle Ordovician (~470Ma), and Late Ordovician (460–450Ma). The Middle Cambrian monzogranites are K-rich, weakly to strongly peraluminous, and characterized by pronounced heavy rare earth element (HREE) depletions, high Sr/Y ratios, low Y concentrations, low primary zircon εHf(t) values (−6.79 to −1.09), and ancient two-stage model (TDM2) ages (1901–1534Ma). These results indicate derivation from partial melting of thickened ancient crustal materials that formed during the amalgamation of the northern SZM and the northern Jiamusi Massif (JM). The Late Cambrian monzonite, quartz monzonite, and monzogranite units are chemically similar to A-type granites, and contain zircons with εHf(t) values of −2.59 to +1.78 and TDM2 ages of 1625–1348Ma. We infer that these rocks formed from primary magmas generated by partial melting of Mesoproterozoic accreted lower crustal materials in a post-collisional extensional environment. The Early–Middle Ordovician quartz monzodiorite, quartz monzonite, monzogranite, and rhyolite units are calc-alkaline, relatively enriched in light REEs (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), depleted in HREEs and high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and contain zircons with εHf(t) values of −7.33 to +4.98, indicative of formation in an active continental margin setting. The Late Ordovician alkali-feldspar granite and rhyolite units have A-type granite affinities that suggest they formed in an extensional environment. A comparison of early Paleozoic magmatic events and Hf isotopic model ages between the northern SZM and the JM indicates that these two massifs have similar histories of Mesoproterozoic and early Paleozoic crustal accretion and reworking, although the SZM contains much older crustal materials than the JM.

Permian alkaline granites in the Erenhot–Hegenshan belt, northern Inner Mongolia, China: Model of generation, time of emplacement and regional tectonic significance

The Central Asian Orogenic Belt (CAOB) is known to be the Earth’s largest Phanerozoic accretionary orogenic belt. Its southern margin, particularly in the southern Mongolia and Inner Mongolia (SMIM), shows an extensive distribution of alkaline granites. Study of these granites could shed light on long-debated hypotheses on the late Paleozoic tectonic evolution of this region. In this work, we performed a detailed zircon age determination and whole rock geochemical analysis on alkaline granites from four granitic plutons in northern Inner Mongolia (Hongol, Saiyinwusu, Baolag, and Baiyinwula). U–Pb zircon dating yielded early Permian ages (ca. 280Ma) for the four plutons. Whole-rock geochemical analyses show chemical characteristics typical of alkaline granites. Coeval alkaline granites from southern Mongolia to northern Inner Mongolia constitute a gigantic (∼900km) Permian (292–275Ma) alkaline granite belt in the southern CAOB. Furthermore, these alkaline granites have whole-rock εNd(t) varying between +3.6 and +6.4 and zircon εHf(t) from +4.9 to +20.3. The geochemical and Nd–Hf isotopic signatures suggest derivation by partial melting of a protolith assemblage dominated by metasomatised mantle, and followed by fractional crystallization in a post-collisional extensional environment.

Geochronological, isotopic and mineral geochemical constraints on the genesis of the Diyanqinamu Mo deposit, Inner Mongolia, China

The Diyanqinamu Mo deposit, located in central Inner Mongolia in China, is a newly discovered Mo deposit. Due to the lack of detailed deposit-scale studies, the metallogenesis of the deposit has been a matter of debate. In this study, petrography, zircon U–Pb geochronology, Sr, Nd and Pb isotopes, and apatite mineral geochemistry have been used to constrain the genesis of the Diyanqinamu Mo deposit.Petrographic evidence indicates that the granites in the Diyanqinamu deposit have experienced fluid exsolution, as demonstrated by the corroded quartz in the porphyritic granites, skeletal quartz in the aplitic granites, and the unidirectional solidification texture (UST) in the apex of the aplitic granites. Sr and Nd isotopic data of the purple fluorite from the fluorite±molybdenite veins, showing the (87Sr/86Sr)i (0.7046–0.7052, avg. 0.7048) and εNd(t) values (2.4–3.8, avg. 3.3) are similar to those of the porphyritic- and aplitic granites. This similarity implies that the Mo ore-forming fluids may have been derived from the granitic magmas. The hypothesis is further supported by the fluorine content (3.79–5.48wt.%, avg. 4.53wt.%) in apatite from the porphyritic granites and the widely distributed fluorite in the hydrothermal alteration mineral assemblages, which indicate that the porphyritic granites contain high F content and the hydrothermal fluids are also F-rich. Petrographic observation also shows that magnetite occurs commonly in the hydrothermal mineral assemblages, indicating that the hydrothermal fluids are characterized by high oxygen fugacity (fO2). It is important to note that the intruding granites are essentially barren, and the Mo is mainly hosted in the andesite and volcaniclastic rocks. In comparison, Pb isotopic compositions of molybdenite are similar to those of both the andesite and K-feldspar from the porphyritic granites, which suggests that both the intruding granites and ore-hosting andesite have contributed to the ore-forming materials. This hypothesis is further supported by the abnormally high Mo for the andesite.Our new evidence suggests that the Diyanqinamu Mo deposit may have formed in a post-collisional extensional environment. Ore-forming materials may have been derived mainly from the granites and minimally from the host rocks. The ore fluids are characterized by high fO2 and being F-rich, and the trigger for the Mo mineralization has been attributed to the reduction of fO2 and/or temperature for the ore fluids.

Recognition of an episode of post-collisional normal faulting in North West Pakistan: Constraints from fission-track analysis

The Sillai Patti carbonatite has been intruded along a NNE–SSW striking, southward dipping Sillai Patti Fault at the contact of the metasediments with the granitic gneiss. Both, time of emplacement and the sense of movement of the blocks on either side of this fault are under considerable dispute. Fission-track analysis on apatite from five carbonatite samples yielded an emplacement age of 29.3±1.2Ma. Comparison of the textures and cooling rates indicates that the granitic gneiss on the southern side of the fault has been uplifted at a faster rate than the carbonatite itself located on the northern side. The abrupt jump from lower fission-track ages in the south to higher fission-track ages in the north across the fault has also been noticed. Moreover, a faster total exhumation of 6.67km during the past 24.4±2.9Ma for the southern hanging wall of the fault has been noticed based on fission-track analysis of zircon as compared to the slower maximum possible total exhumation of ≤1.67km derived from the fission-track analysis of apatite during the past 29.3±1.2Ma for the northern footwall. These facts reveal that the Sillai Patti Fault, a southward extension of the Main Mantle Thrust (MMT), showing an apparently thrust sense of movement on local scale at Sillai Patti, has behaved as a normal fault on regional scale further to the north at the contact of the Indian Plate with the Kohistan Island Arc. During this episode of normal faulting the Kohistan Island Arc slid down and to the north along the MMT relative to the Indian Plate. Recognition of an episode of normal faulting during Oligocene on the basis of this study clearly demonstrates that carbonatitic magmatism took place in the region in a post collisional extensional environment along normal fault(s).

Depleted-mantle source for the Ulungur River A-type granites from North Xinjiang, China: geochemistry and Nd–Sr isotopic evidence, and implications for Phanerozoic crustal growth

The Ulungur River granites from North Xinjiang, China, display two petrological trends: (1) a peralkaline trend, composed of alkali amphibole- and alkali pyroxene-bearing granites and dykes, and (2) an aluminous trend, represented by alkali granites, monzogranites and aluminous granite dykes within the peralkaline granites. Rb–Sr whole-rock isochron dating yielded an age of 300 Ma for the emplacement of the peralkaline granites and 270 Ma for the aluminous granites. The Ulungur River granites share all the geochemical features common to A-type granites of the world. The Ulungur River granites are post-orogenic and emplaced in a post-collisional extensional environment. The peralkaline and aluminous rocks have indistinguishable Nd isotopic compositions. They have unusually positive and relatively uniform ϵ Nd( T) values, ranging from +5.1 to +6.7 for the peralkaline rocks at 300 Ma, and from +5.5 to +5.9 for the aluminous rocks at 270 Ma. The initial 87Sr/ 86Sr ratios cannot be determined precisely by the whole-rock isochron technique mainly because most samples are highly radiogenic and have high Rb/Sr ratios. In addition, the large variation in Sr isotopic compositions can partly be explained by crustal contamination during magma differentiation. The Nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long-lived depleted mantle reservoir. The formation of the Ulungur River A-type granites is hypothesized as follows: (1) a depleted mantle was first metasomatised, then subjected to partial melting leading to the generation of basic magmas now represented by the Kalatongke mafic–ultramafic complex in the Ulungur River area; (2) the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite, hornblende, plagioclase, potassium feldspar, and other accessory phases such as apatite and ilmenite, with the residual magmas forming the peralkaline granites. During the magmatic differentiation, the liquids were probably contaminated by crustal materials as evidenced by Nb depletion and Nd–Sr isotopic systematics. On the other hand, the formation of aluminous granites involved a replenishment of new mantle-derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases (hornblende and biotite). The petrological evolution of the Ulungur River granites is thus from peralkaline to calc-alkaline, and not from calc-alkaline to peralkaline as more often observed elsewhere. The Ulungur River granites, in common with the voluminous granitoids occurring in the Central Asian Orogenic Belt, represent a significant juvenile addition of the continental crust in the Phanerozoic.