Weighted Mean Age Research Articles

Zircon geochronology and paleomagnetism of an Archean harzburgite intrusion, eastern Bighorn Mountains, Wyoming

A harzburgite intrusion, which is part of the trailside mafic complex) intrudes ~2900-2950 Ma gneisses in the hanging wall of the Laramide Bighorn uplift west of Buffalo, Wyoming. The harzburgite is composed of pristine orthopyroxene (bronzite), clinopyroxene, serpentine after olivine and accessory magnetite-serpentinite seams, and strike-slip striated shear zones. The harzburgite is crosscut by a hydrothermally altered wehrlite dike (N20°E, 90°, 1 meter wide) with no zircons recovered. Zircons from the harzburgite reveal two ages: 1) a younger set that has a concordia upper intercept age of 2908±6 Ma and a weighted mean age of 2909.5±6.1 Ma; and 2) an older set that has a concordia upper intercept age of 2934.1±8.9 Ma and a weighted mean age 2940.5±5.8 Ma. Anisotropy of magnetic susceptibility (AMS) was used as a proxy for magmatic intrusion and the harzburgite preserves a sub-horizontal Kmax fabric (n=18) suggesting lateral intrusion. Alternating Field (AF) demagnetization for the harzburgite yielded a paleopole of 177.7 longitude, -14.4 latitude. The AF paleopole for the wehrlite dike has a vertical (90°) inclination suggesting intrusion at high latitude. The wehrlite dike preserves a Kmax fabric (n=19) that plots along the great circle of the dike and is difficult to interpret. The harzburgite has a two-component magnetization preserved that indicates a younger Cretaceous chemical overprint that may indicate a 90° clockwise vertical axis rotation of the Clear Creek thrust hanging wall, a range-bounding east-directed thrust fault that accommodated uplift of Bighorn Mountains during the Eocene Laramide Orogeny.

The Tongkuangyu Cu Deposit, Trans-North China Orogen: A Metamorphosed Paleoproterozoic Porphyry Cu Deposit

AbstractThe Tongkuangyu copper deposit in the Zhongtiaoshan region, southern Trans-North China orogen, is hosted by a poorly constrained sequence of Paleoproterozoic volcano-sedimentary (quartz-sericite schist and biotite schist) and granitic rocks that have been metamorphosed to lower greenschist facies and variably deformed. The deposit has previously been proposed to be either a porphyry-type or a sediment-hosted stratiform Cu deposit, and its age of formation has been debated.The quartz-sericite schist is interpreted to be a felsic crystal tuff and consists of angular quartz crystals in a fine-grained sericite-altered matrix. Two quartz-sericite schist samples yielded zircon U-Pb upper concordia intercept ages of 2512 ± 12 (2σ, mean square of weighted deviates [MSWD] = 0.19) and 2335 ± 16 Ma (2σ, MSWD = 0.80). Biotite schist, which is interleaved locally with the quartz-sericite schist and is interpreted to be a basaltic-andesitic sill, yielded a younger zircon U-Pb upper concordia intercept age of 2191 ± 10 Ma (2σ, MSWD = 1.7). Five samples of granodiorite and granodiorite porphyry that intruded the schist sequence yielded similar zircon U-Pb ages, with a weighted mean upper concordia intercept age of 2182 ± 7 Ma (2σ, MSWD = 1.3). These results suggest that the volcanic sequence was deposited between ∼2.5 and 2.3 Ga and was intruded by basaltic-andesitic sills and a suite of granodiorite and granodiorite porphyry intrusions at ∼2.19 to 2.18 Ga.Two stages of copper mineralization are interpreted to have formed after pervasive sericite alteration of the felsic volcanic rocks. Stage 1 mineralization includes disseminated and deformed quartz veinlets containing chalcopyrite ± pyrite ± magnetite ± molybdenite associated with biotite ± K-feldspar alteration in granodiorite porphyry and schist. Stage 2 comprises undeformed quartz-chlorite-carbonate veins with bornite ± chalcopyrite ± magnetite associated with local chlorite and silicic alteration. Allanite crystals intergrown with chalcopyrite in the granodiorite porphyry yielded an approximate concordia U-Pb age of 2115 ± 31 Ma (2σ, MSWD = 2.3). Two molybdenite samples in a deformed quartz-chalcopyrite-molybdenite vein yielded Re-Os model ages of 2106 ± 9 and 2089 ± 9 Ma (2σ), consistent with previously published results.Hydrothermal monazite grains with Cu-Fe sulfide inclusions in the granodiorite porphyry, quartz-sericite schist, and undeformed chlorite-bornite-quartz veins yielded much younger U-Pb upper concordia intercept ages of 1832 ± 16 (2σ, MSWD = 0.48), 1810 ± 14 (2σ, MSWD = 0.92), and 1809 ± 12 Ma (2σ, MSWD = 0.38), respectively. The results are in agreement with four Re-Os model ages for pyrite mineral separates from undeformed quartz-sulfide veins, which yielded a weighted mean age of 1807 ± 4 Ma (2σ, n = 4, MSWD = 0.42). In contrast, hydrothermal rutile crystals in the quartz-sericite schist and biotite schist yielded a range of roughly concordant ages between 2.1 and 1.8 Ga, reflecting isotopic disturbance.We interpret these results to indicate original copper mineralization at ∼2.1 Ga that is significantly later than the granodiorite (∼2.18 Ga) and schists (∼2.5–2.2 Ga), followed by hydrothermal remobilization and metamorphism at ∼1.8 Ga. The metavolcanic and granodiorite porphyry host rocks, alteration styles, and disseminated and veinlet form of the earlier mineralization are strongly reminiscent of porphyry Cu deposits, and ages of ∼2.1 Ga have been reported for one intrusion and three volcanic rock samples from the district. The Tongkuangyu, therefore, represents one of the oldest known porphyry copper deposits. Remobilization of copper occurred at ∼1.8 Ga during the Zhongtiao orogeny.

Geochemistry, geochronology and evolution of Paleoproterozoic granitoid gneisses in the Khondalite Belt, North China Craton

The studied granitoid gneisses from the Daqingshan complex in the Khondalite Belt of the North China Craton consist of tonalitic, trondhjemitic, and granodioritic (TTG) gneisses, as well as monzogranitic and granitic (MG) gneisses. The respective melts forming these granitoid gneisses are geochemically similar. The granitoid gneisses are generally enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, Nd, La, Ce and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. Chondrite-normalized rare earth elements (REE) patterns indicate that both TTGs and MG gneisses illustrate various fractionation with (La/Yb)N ratios of 7.43–58.29 and 3.59–87.82, respectively. However, most TTG gneisses show positive Eu anomalies, and relatively negative Th and positive Sr anomalies, while MG gneisses have opposite characteristics. These geochemical characteristics suggest that TTG gneisses were derived from a magma source at higher pressures than that to have formed MG gneisses. Some TTG gneisses yield high Sr/Y (36–139) and (La/Yb)N ratios, indicating a subduction-related affinity. The whole series of granitoid gneisses from the Daqingshan complex belong to fractionated calc-alkaline I-type granitoids, suggesting their igneous protoliths akin to island arcs and continental magmatic arcs. LA-ICP-MS U–Pb dating of zircons from four representative granitoid samples indicates that tonalite and granodiorite exhibit weighted mean ages of 2416 ± 12 Ma and 2348 ± 10 Ma, respectively, whereas trondhjemite provides two groups of zircon ages yielding weighted mean ages of 2437 ± 15 Ma and 1876 ± 3 Ma, respectively. The studied granitic gneiss reveals an upper intercept of 1947 ± 45 Ma. Integration of cathodoluminescence images, Th/U ratio, and REE patterns of zircons indicates that all zircons of granitoid gneisses have a magmatic origin. Therefore, three major episodes of granitic magmatism at ca. 2.35–2.42, 1.95, and 1.88 Ga can be inferred. The ages of 1.95 and 1.88 Ga are coeval with widespread metamorphic events recorded by mafic and pelitic granulites in the Khondalite Belt. The Khondalite Belt could have undergone long-term arc-continent accretion along the southern margin of the Yinshan Block during early to late Paleoproterozoic time, and subsequent continental collision between the Yinshan and Ordos Blocks occurred at ca 1.95–1.88 Ga.

Metallogenesis of stratiform Cu mineralization in the Dabaoshan polymetallic deposit, Northern Guangdong Province, South China

The Dabaoshan Cu–Pb–Zn–Mo–W polymetallic deposit in northern Guangdong Province, South China, lies in the southern part of the Qin–Hang porphyry–skarn CuMo ore belt and hosts both porphyry Mo and stratabound skarn Cu ores. The genesis of the Cu ore has been ascribed to typical skarn ore formed by the metasomatism of Devonian carbonate rock layers, or to a sediment-rock-hosted massive sulfide deposit formed by marine exhalation. In this paper, we report the ages of porphyry Mo and stratiform Cu ores and dacitic porphyry determined with molybdenite ReOs, sericite Ar/Ar, and zircon UPb methods, respectively. Nine samples from the granodiorite porphyry Mo ores yielded ReOs model ages ranging from 165.7 ± 2.3 to 163.4 ± 2.4 Ma, with a weighted mean age of 164.8 ± 0.8 Ma (MSWD = 0.52). Sericite from two samples of stratiform Cu ores yielded ages of 166.6 ± 1.6 Ma (DB011) and 171.7 ± 2.0 Ma (DB020), with corresponding reverse isochron ages of 167.0 ± 2.0 Ma and 161.7 ± 1.7 Ma, respectively. The age data determined from ReOs analyses of molybdenite and 40Ar/39Ar dating of sericite are in good agreement with each other, indicating that the porphyry Mo and stratiform Cu potentially originated from the same ore system and may be genetically associated with Jurassic granitic granodiorite porphyry. In this study, magmatic zircon weighted mean ages of 434.3 ± 1.0 Ma (DB009), 431.5 ± 0.12 Ma (DB022), and 427.0 ± 0.87 Ma (DB031) from three samples for the Dabaoshan dacitic porphyry indicate that the mineralization in the Dabaoshan deposit is mainly related to the invasion of Jurassic granitic magma, not to Silurian subvolcanic rocks. Based on the integrated geological, geophysical, and geochemical data, we proposed the Dabaoshan porphyry–skarn Cu–Pb–Zn–Mo–Wpolymetallic deposit to be part of the Qin–Hang porphyry–skarn CuMo ore belt that formed during the low-angle Jurassic subduction of the Paleo-pacific plate under Eurasian continent.

In search of the inland Carnian Pluvial Event: Middle–Upper Triassic transition profile and U–Pb isotopic dating in the Yanchang Formation in Ordos Basin, China

The Carnian Pluvial Event (CPE) in the early Late Triassic is a global climate change. The CPE is not only recorded in the ocean but also on the continent. It has been well‐documented in the marine environments in Tethys and boreal areas. However, it is less reported for the terrestrial. As a global precipitation event, the CPE on the supercontinent Pangaea is related directly to the sudden increase in argillaceous transportation in the marine carbonate platform. Here, we report a chronological study of the Yunmeng profile in south‐eastern Ordos Basin, China where the continuous lacustrine deposits developed with several tuff interlayers. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) U–Pb isotopic dating of zircons from three bentonite samples show weighted mean ages of 234.1 ± 2.4 Ma, 234.8 ± 2.1 Ma, and 236.0 ± 1.7 Ma, respectively. Based on the review of isotopic dating, sedimentary and palaeontological characteristics of the Triassic Yanchang Formation, the Middle and Upper Triassic boundary is located at the bottom of Chang 7 oil layer (YC7) in the Yanchang Formation at the Yunmeng profile in the Ordos Basin. Therefore, the Yunmeng profile provides a potential case for understanding the inland CPE and might link the terrestrial and oceanic systems in the Carnian.

Low–pressure metamorphism of mafic granulites in the Chinese Altay orogen, NW China: P–T path, U–Pb ages and tectonic implications

Mafic granulites occur as massive lenses within host paragneisses at Wuqiagou area in the southern Chinese Altay orogen. They comprise a suite of fine–grained and coarse–grained mafic granulites, both characterized by an assemblage of orthopyroxene, clinopyroxene, hornblende, biotite, plagioclase, quartz, Fe–Ti oxides, minor K–feldspar, and accessary apatite and zircon. The peak– and post–peak metamorphism evolution is respectively represented by orthopyroxene–clinopyroxene–bearing assemblages, and the clinopyroxene–plagioclase–bearing or hornblende–biotite–plagioclase–bearing associations. Thermobarometries and pseudosection modelling constrain the peak P–T conditions at ~5.5 kbar and 770–820 °C, with gradients of 40–45 °C/km for the this stage, and at ~3.0 kbar and 630–670 °C for the retrograde evolution, indicative of a possible P–T path with a post–peak cooling and slight decompression process. LA–ICP–MS and SHRIMP zircon U–Pb age dating results of two mafic granulite samples show weighted mean ages of 275 ± 1.6 Ma and 271 ± 3.2 Ma, respectively. These ages were interpreted to represent the timing of the low–pressure granulite–facies metamorphic event in this region, suggesting relatively high thermal conditions in the late Paleozoic. This metamorphism was highly coeval with the deep–derived mafic intrusions and Tarim mantle plume activity (~275 Ma). Accordingly, we concluded that the Permian regional thermal anomaly in this region might be closely related to the Tarim mantle plume activity.

Petrogenesis and tectonic setting of the Middle Devonian Beitashan Formation volcanic rocks in the northern East Junggar, NW China: Insights from geochemistry, zircon U–Pb dating, and Hf isotopes

The Devonian volcanic rocks are widely distributed in the East Junggar (Xinjiang, NW China), preserving important magmatic records for investigating Palaeozoic tectonic evolution. The Beitashan Formation is mainly composed of pyroclastic rocks and sedimentary rocks in the lower section and intermediate–basic volcanic lava and pyroclastic rocks in the upper section. In this paper, we present geochronological, geochemical, and in situ Lu–Hf isotopic data for Beitashan Formation volcanic rocks from the East Junggar. Zircon U–Pb analyses from a basalt sample by laser ablation inductively coupled plasma mass spectrometry yielded a weighted mean age of 387.8 ± 4.5 Ma, indicating that it was generated in the Middle Devonian. The basic–intermediate volcanic rocks are tholeiitic to calc‐alkalic and are characterized by MgO (2.29–5.91 wt.%), low TiO2 (0.75–1.36 wt.%), and relatively high Al2O3 (15.82–16.73 wt.%). Furthermore, they display enrichment in light rare‐earth elements and large‐ion lithophile elements (such as Ba and Sr) and depletion in Nb, Ta, and Ti, along with slightly positive Eu anomalies (δEu = 0.83–1.15) and Ce anomalies (δCe = 1.00–1.11). In addition, in situ zircon Hf isotopic analyses from basalt samples show εHf(t) values ranging from +4.33 to +15.2, indicating a depleted mantle source with minor crustal contamination. They also show relatively low Nb/Zr (0.04–0.06), Th/Yb (0.67–3.83), and Ba/La (6.79–67.53) and high Ce/Th (7.71–18.94), Ba/Rb (21.39–294.28), and Ba/Th (31.42–486.58). The geochemical data suggest these rocks were likely derived from 10% to 30% partial melting of a garnet‐rich with minor spinel‐bearing Iherzolite depleted mantle metasomatized by slab‐derived fluids, with crustal contamination. Combined with the geological setting and previous studies, we suggest that the northern East Junggar was in a northward‐subducted island‐arc setting in the Devonian.

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

AbstractWe report five new Rb‐Sr muscovite‐based isochron ages, which are the first to constrain the timing of amphibolite‐facies mylonitization of the Alpine Schist in the Southern Alps of New Zealand. The ages range from 13.1 ± 4.3 to 8.9 ± 3.2 Ma (2σ uncertainties) for mylonite directly above the Alpine Fault. The weighted mean age of 10.74 ± 0.57 Ma is within uncertainty of a published 40Ar/39Ar illite/mica upper‐intercept age of 11.5 ± 0.5 Ma measured at the same locality. The end of amphibolite‐facies mylonitization occurred at metamorphic conditions of ~560–570 °C and ~0.9–1.1 GPa as derived from pseudosection analysis in the NCTiKFMASH system. We interpret Miocene metamorphism to reflect transpressional crustal thickening and formation of a thick crustal root supporting early Southern Alps topography at or prior to 10.74 ± 0.57 Ma. Additional ~2–1 Ma Rb‐Sr biotite, 40Ar/39Ar muscovite, and 40Ar/39Ar biotite ages reflect isotopic closure during rapid cooling along the Alpine Fault in the Pleistocene. The Miocene mylonitization ages and the Pleistocene cooling ages define a distinct two‐stage cooling and exhumation history for the Alpine Schist with initial cooling of ~10 °C/Myr and exhumation rates of 2–4 km/Myr. Final cooling since ~2 Ma was >100 °C/Myr at exhumation rates of ~5–6 km/Myr. We interpret the two‐phase cooling history by movement of the mylonite through a strongly nonlinear thermal structure. An older 60.5 ± 0.7 Ma metamorphic event is also preserved as a Rb‐Sr crystallization age of a predeformational muscovite‐plagioclase assemblage in a sheared pegmatite.

Imbrium Age for Zircons in Apollo 17 South Massif Impact Melt Breccia 73155

AbstractThe Apollo 17 poikilitic impact melt breccias were initially interpreted to be directly related to the Serenitatis basin formation. Here we present petrological, geochemical, and U‐Pb geochronological investigations of the poikilitic impact melt of Apollo 73155,69 which was sampled from the South Massif of the Taurus‐Littrow valley. Nanoscale secondary ion mass spectrometry analyses of well‐preserved poikilitic zircons from sample 73155,69 yield a uniform 207Pb/206Pb age population with a weighted mean age of 3921 ± 14 Ma. The zircons have distinctly high concentrations of Y (3279−6347 ppm) and Th (254−302 ppm) compared with other Apollo 17 zircons. These characteristics, together with their textures, are remarkably similar to those of zircons from high‐Th melt breccias from Apollo 12 samples and lunar meteorite Sayh al Uhaymir 169 attributed to the formation of the high‐Th deposits of the Imbrium basin. We therefore propose an Imbrium provenance for melt component in 73155,69. Our results provide new evidence for components of Imbrium‐type ejecta occurring among the South Massif breccias. Consequently, the Apollo 17 poikilitic impact melt breccias may not represent Serenitatis ejecta.

The age and paleoenvironment of a late Miocene floodplain alongside Lake Pannon: Rodent and mollusk biostratigraphy coupled with authigenic 10Be/9Be dating in the northern Danube Basin of Slovakia

We present new age constraints for the late Miocene Volkovce Formation of Slovakia, alluvial facies that accumulated adjacent to Lake Pannon. Facies analysis and authigenic 10Be/9Be dating was undertaken at the Bernolákovo and Triblavina localities while the latter outcrop also yielded additional small mammal and mollusk assemblages as well as remains of flora. The results of seven radiometric dating ages yielded a weighted mean age of 9.65 ± 0.61 Ma for the Triblavina outcrop and 8.85 ± 0.32 Ma for the Bernolákovo outcrop. The small mammal assemblage at the Triblavina outcrop is indicative of the lowermost MN11 zone, which in the Western European standard definition suggests an age of <8.9 Ma. The ~0.1 Ma inconsistency between absolute dating and biostratigraphic dating at the Triblavina outcrop might be explained either by the variation of initial 10Be/9Be ratio in the depositional environment or/and by diachronous development of the mammal faunas across the Europe. The mollusk assemblage at this outcrop indicates the paleoenvironment of freshwater lakes affected by rivers with evidence for a nearby riparian forests; it may be roughly correlated with Papp's lithostratigraphic units G and H defined for the Pannonian stage in the Vienna Basin. Our results provide a detailed insight to the late Miocene stratigraphy and terrestrial environments alongside the Lake Pannon shoreline.

Origin of mafic intrusions in the Micangshan Massif, Central China: Implications for the Neoproterozoic tectonic evolution of the northwestern Yangtze Block

The mafic intrusions exposed in the Micangshan Massif preserve important information pertaining to the Neoproterozoic tectonic evolution along the northwestern margin of the Yangtze Block. In this paper, we report new LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopes and whole-rock compositions for mafic sills in the Micangshan Massif to understand their petrogenesis and supplement the regional tectonic evolution. Xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 ± 4 Ma and 858 ± 5 Ma, respectively, suggesting the maximum emplaced age of the mafic sills was at ca. 860 Ma. A granitic dyke sample which intruded into the mafic sills, yields a crystallization age of 799 ± 4 Ma, implying the minimum emplacement of the mafic sills was at ca. 800 Ma. These results indicate that the mafic sills were formed within the span of ca. 860–800 Ma. Geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate FeOtotal and TiO2 contents, as well as moderate to high MgO contents and Mg# values. They display distinct enriched light rare earth element (LREE) patterns, variable remarkable enrichment in large-ion lithophile elements (LILEs; e.g., Cs and Ba), and evident depletion of high-field strength elements (HFSEs; e.g., Nb, Ta and P), resembling signatures of typical subduction-related basaltic rocks. In addition, these sills exhibit comparatively high Nb/Ta, moderate Nb/Yb, as well as low Nb/La, Th/Yb, and Nb/Y ratios, indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source. These observations, together with regional tectonics, suggest that these mafic sills in the Micangshan Massif might be formed in a subduction-related continental arc setting. Therefore, combining our new data with available results from previous studies in the adjacent regions, our study supports the dominant subduction-related tectonics along the northwestern margin of the Yangtze Block during the Early Neoproterozoic time.

In-situ U–Pb dating of monazite, xenotime, and zircon from the Lantian black shales: Time constraints on provenances, deposition and fluid flow events

The Lantian formation, located in eastern South China block (SCB), forms the lower sequence of the Ediacaran period, at the transition from Proterozoic to Paleozoic. The fossils of Lantian formation have been proposed to predate the Avalon biota (about 579–565 Ma), the earliest known fossil assemblages with complex morphologies, based largely on stratigraphy. Mineral chemistry and in-situ U–Pb dating of monazites, xenotimes, and zircons from the black shale in fossil-rich zone are studied for the first time to provide critical age constraints for provenance, early diagenesis and fluid flow events. Detrital zircons and monazites give age populations similar to that of the Doushantuo formation, indicative of the same provenances for Ediacaran basins throughout the SCB. The low-Y monazites formed during early diagenesis yield a weighted mean age of 612 ± 29 Ma (n = 9, MSWD = 3.1). Age spectrum of analyzed monazites demonstrated that the sedimentation started from at least around 645 Ma. Subsequently, the overprinting events could be recorded by xenotimes and zircons precipitated from fluids during late diagenesis. Age results from xenotimes ranging from 441 Ma and 585 Ma were contemporaneous with the timing of collision of SCB with Gondwanaland, and a weighted mean age of 394.8 ± 3.7 Ma (n = 23, MSWD = 2.8) obtained from diagenetic zircons could be correlated to the post-tectonic granites widespread on the SCB. The pre-existing monazites generally remain pristine, and the multi-staged fluids were locally interacted with the samples to form Th-rich domains in monazites.

A multi‐method approach to dating the burial and skeleton of Kiacatoo Man, New South Wales, Australia

ABSTRACTKiacatoo Man, a large, rugged Aboriginal adult buried in the Lachlan riverine plains of southeastern Australia, was discovered in 2011. Laser‐ablation uranium series analysis on bone yielded a minimum age for the burial of 27.4 ± 0.4 ka (2σ). Single‐grain, optically stimulated luminescence ages on quartz sediment in which the grave had been dug gave a weighted mean age of 26.4 ± 1.5 ka (1σ). Luminescence samples from the grave infill and from sediment beneath the grave exhibit overdispersed dose distributions consistent with bioturbation or other disturbance, which has obscured the burial signal. The overlap between the minimum (U‐series) and maximum (luminescence) ages places the burial between 27.0 and 29.4 ka (2σ). Luminescence ages obtained from the channel belt of between 28 ± 2 and 25 ± 3 ka indicate that fluvial sedimentation was occurring before the Last Glacial Maximum, which is consistent with the broader geomorphic setting. Together, these results are internally and regionally consistent, and indicate that Kiacatoo Man was one of the more ancient individuals so far identified in Australia. His remains are important to our understanding of patterns of biological variation and other processes that have shaped people in the Murray‐Darling Basin through time. Copyright © 2019 John Wiley &amp; Sons, Ltd.

Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet

In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158–161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cretaceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive eHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of eHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

Separating multiple episodes of partial melting in polyorogenic crust: An example from the Haiyangsuo complex, northern Sulu belt, eastern China

Abstract The exotic Haiyangsuo complex is structurally part of the Sulu belt but its contact relationship with surrounding Sulu gneisses is unexposed and therefore unknown, making its affinity uncertain. It comprises gneisses with in-source leucosomes that host minor metabasite bodies; both are cut by leucogranite dikes. In this study, we determine the timing and petrogenesis of leucosomes and leucogranites and assess the tectonic affinity of the complex based on data from gneisses and metabasites. Most zircon from gneisses and leucosomes has oscillatory-zoned cores with CL-bright overgrowth rims, but some has CL-dark cores or mantles between cores and rims. CL-dark and bright zircon yield weighted mean ages of ca. 1817–1812 Ma. CL-dark zircon has flat heavy rare earth element (HREE) patterns and crystallization temperatures of 829–875 °C, suggesting metamorphic growth, whereas rims have steep HREE patterns but a similar range of crystallization temperatures, suggesting growth from anatectic melt; εHf (t = 1813 Ma) of –18.3 to –10.8 indicates a North China Craton source. Magmatic zircon from metabasites yields ages of ca. 825 Ma, similar to those of scattered metabasite occurrences in the North China Craton. Paleoproterozoic zircon cores were scavenged during magma ascent. By contrast, zircon cores from the leucogranites yield concordant dates of 776–701 Ma, consistent with protolith ages in the Sulu belt, whereas overgrowth mantles and rims yield weighted mean ages of ca. 220 Ma and 162 Ma, respectively. Both mantles and rims host multiphase solid inclusions, representing former melt, suggesting anatexis and crystallization of zircon first during initial decompression and then during orogenic collapse of the Sulu belt; whole-rock Nd and Sr isotope compositions implicate the Sulu belt gneisses as the source of these melts. Our interpretation of these data is that the Haiyangsuo complex has an early geologic history similar to the Jiaobei terrane from the southeastern part of the North China Craton and was incorporated into the Sulu belt during Triassic collision of the Yangtze and North China Cratons. The two stages of melting relate to Upper Triassic early exhumation and Upper Jurassic late-stage orogenic collapse, during which the leucogranite magma was derived from a source similar to one elsewhere in the Sulu belt such as the subducted Yangtze Craton and not the North China Craton. This shows that during continental collisions, crust from the upper plate may be dragged into the subduction channel, deformed, and subsequently exhumed in association with partial melting of the crust.

The tectonic evolution of the Changchun-Yanji suture zone: Constraints of zircon U-Pb ages of the Yantongshan accretionary complex (NE China)

The Changchun-Yanji suture zone is composed of a sequence of high-pressure metamorphic rocks. It lies to the southern margin of the Jilin-Heilongjiang high-pressure metamorphic belt and separates the Jiamusi-Khanka Block to the north from the North China Craton. In this study, to understand the nature of the Changchun-Yanji suture zone, we studied four samples in diverse lithology (i.e., two chlorite schists, one felsic dike sample, and one piemontite schist) from the Yantongshan Complex. These rocks have been intensely deformed without preservation of original sedimentary features. Integrated evidence from high-pressure metamorphic minerals and zircon U-Pb age results suggests that most of the Paleozoic strata in this region might be part of a tectonic mélange. Phengite in the piemontite schist contains high Si (≥3.304 based on 11 oxygens), indicative of high-pressure metamorphism. The piemontite schist contains 6 zircon age populations from 2505 ± 40 to 231 ± 10 Ma. The 22 youngest zircon grains define a weighted mean age of 237 ± 5 Ma, placing an upper limit on the depositional time of the protolith of the piemontite schist. The felsic dike intruded into the metasedimentary rocks gave concordant ages with a weighted mean age of 217 ± 5 Ma, providing a minimum emplacement age for the Yantongshan Complex. We conclude from these data that the final collision between the Jiamusi-Khanka Block and North China Craton along the Changchun-Yanji suture zone occurred before the Late Triassic. Moreover, this closure time is at least 30 Myr later than closure along the Solonker-Xar Moron suture in the late Permian. As such, we consider that the Changchun-Yanji suture zone was not related to amalgamation of the Siberian and North China cratons, but was the result of the subduction of the Paleo-Pacific Plate.

Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge, Saxothuringian Zone

A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.

Permian ultrahigh–temperature reworking in the southern Chinese Altai: Evidence from petrology, P–T estimates, zircon and monazite U–Th–Pb geochronology

The Chinese Altai orogen formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), and the study on the metamorphism will provide novel and robust constraints on its tectonic evolution. In this study, we investigate our newly recognized garnet–orthopyroxene–cordierite granulites at Wuqiagou area in the southern Chinese Altai. Detailed petrographic study and P–T estimates suggest four distinct metamorphic stages of mineral assemblages: (1) pre–peak (M1) stage containing the spinel–cordierite–bearing association or biotite–plagioclase–quartz–bearing inclusion–phase assemblage, with P–T conditions of 3.0–4.0 kbar/700–750 °C; (2) peak ultrahigh–temperature (UHT) (M2) stage represented by relatively coarse–grained garnet–orthopyroxene–cordierite–bearing porphyroblastic assemblage, with high–Al2O3 contents (up to ∼8.7 wt%) in orthopyroxene and P–T conditions of ∼8.0 kbar/∼980 °C; (3) post–peak high–temperature granulite facies (M3) stage consisted of orthopyroxene–cordierite and cordierite–quartz corona assemblages, formed during cooling and moderate decompression; and (4) post–peak upper amphibolite facies (M4) stage represented by retrograde biotite–plagioclase–quartz intergrowths. These four discrete metamorphic stages define an anticlockwise P–T path involving a post–peak moderate decompression followed by nearly isobaric cooling process. LA–ICP–MS U–Pb age dating results of metamorphic zircons for UHT samples show two weighted mean ages of ∼390 Ma and ∼280 Ma. We propose that the M1 stage might occur in the middle Devonian, whereas the near–peak UHT stage probably occurred in the early Permian. The Permian UHT metamorphism was further supported by the monazite U–Th–Pb dating results (287.9 ± 2.1 Ma), reflecting a prominent HT–UHT reworking event in the late Paleozoic. We proposed that the Permian UHT reworking event in the southern Chinese Altai probably occurred in a post–orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of deep–derived magma as a result of the Tarim mantle plume activity.

Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress

Abstract Soft-sediment deformational structures associated with paleoseismicity (e.g., planar clastic dikes) exist within Upper Cretaceous Mesaverde Group strata in the Laramide Elk Basin anticline, northern Bighorn Basin (Wyoming, USA). Retrodeformation of the Elk Basin anticline to a horizontal Mesaverde Group position indicates that all basement offset is removed and that clastic dikes exhibit a dominant northeast trend. The trend of clastic dikes corresponds to the interpreted northeast-southwest direction of early Laramide layer-parallel shortening, suggesting that the development of clastic dikes recorded initiation of basement deformation and Laramide tectonism. To determine the timing of clastic dike development, we present zircon U-Pb geochronology from the stratigraphically lowest sand-source bed generating upwardly injected clastic dikes and a volcanic bentonite bed (Ardmore bentonite) above the stratigraphic interval containing clastic dikes. Weighted mean ages bracket clastic dike development between 82.4 and 78.0 Ma. Our results imply initiation of basement deformation ∼8–15 m.y. prior than other estimates in the Bighorn Basin. Therefore, we interpret the development of clastic dikes in the Elk Basin anticline to represent an initial phase of Laramide tectonism associated with an applied end load stress transmitted from the southwestern North American plate margin in response to the collision of the conjugate Shatsky Rise oceanic plateau ca. 90–85 Ma. Results demonstrate how sedimentary responses in the foreland can be used to understand tectonic processes at plate boundaries and provide spatial-temporal parameters for models of Laramide deformation.