Abstract The Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA‐ICP‐MS zircon U‐Pb analyses suggest that syenogranite has a weighted mean 206 Pb/ 238 U age of 86±1 Ma (mean square weighted deviation=0.37), which is in accordance with the muscovite Ar‐Ar age (85±1 Ma) of Cu‐Au ore‐bearing skarns and the zircon U‐Pb age (84±1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu–Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al‐bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO 2 (65.10–70.91wt%), K 2 O (3.44–5.17wt%), and total K 2 O+Na 2 O (7.13–8.15wt%), and moderate contents of A1 2 O 3 (14.14–16.45wt%) and CaO (2.33–4.11wt%), with a Reitman index (σ 43 ) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P 2 O 5 contents show a negative correlation with SiO 2 , whereas Pb contents show a positive correlation with SiO 2 . Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc–alkaline metaluminous I–type granite. It is enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies (δCe=1.00–1.04). The relatively low initial 87 Sr/ 86 Sr ratios of 0.7106 to 0.7179, positive ε Hf ( t ) values of 1.0 to 4.1, and two‐stage Hf model ages ( T DM2 ) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The ( 143 Nd/ 144 Nd) t values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its ε Nd ( t ) values range from –10.17 to –6.10, its ( 206 Pb / 204 Pb) t values range from 18.683 to 18.746, its ( 207 Pb / 204 Pb) t values range from 15.695 to 15.700, and its ( 208 Pb / 204 Pb) t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the post‐collision extension of the Qiangtang and Lhasa terranes.
Cumulate rocks of the Upper Main Zone and Upper Zone (UUMZ) of the Bushveld Complex, South Africa, contain the world's major resources of Fe–Ti–V ± P, hosted in Ti-magnetite and apatite, and are commonly considered as having crystallized from the last major injection of magma into the magma chamber. In this study, we present the petrography, modal proportions, whole-rock major element chemistry (260 samples), electron microprobe data (∼10 000 analyses for plagioclase, olivine, and pyroxene), and compiled analyses of Cr in magnetite (239 samples) for the UUMZ sampled over 2·1 km of the Bierkraal drill cores in the western limb of the Complex. The UUMZ section exhibits a broad normal fractionation trend upwards, but a series of reversals to more primitive anorthite contents in plagioclase, Mg# in pyroxenes and olivine, Cr in whole-rocks and Cr in magnetite separates are observed, accompanied by the appearance or disappearance of various minerals. Anorthosite or leucogabbro layers are closely linked to these reversals; the reversals in An % of plagioclase are used as boundaries to divide the UUMZ into 18 cycles. These cycles are interpreted as indications of magma chamber replenishment by plagioclase-laden magmas (up to 20 vol. % plagioclase) and are also marked by spikes in Cr content. In addition, abundant Fe–Ti oxide-bearing plagioclase-rich rocks are identified in the lower half of the UUMZ. These have crystallized from a hybrid melt produced by the mixing of a new plagioclase-bearing magma batch and the resident magma. Further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the UUMZ. The Bushveld UUMZ therefore grew by multiple emplacements of crystal-laden magmas coming from deep-seated chambers. Slow cooling in a shallow chamber explains the systematic bottom-up compositional evolution in the cumulate pile within individual cycles. The residual melt reached silicate liquid immiscibility soon after the saturation of apatite. Thereafter, segregation of conjugate Fe-rich and Si-rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in Fe–Ti oxides, whereas plagioclase mode increases in each apatite-bearing cycle. A comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the Bushveld limbs demonstrates the possible connectivity between the western and eastern Upper Zone but indicates notable differences from the Bellevue section of the northern limb.
Abstract Seismic observations have revealed two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean in the lowermost mantle 1–4 . However, the detailed morphology and stability of the LLSVPs and their interaction with the surrounding mantle remain unclear. Here we show that, whereas the maximum height of the Pacific LLSVP is ~500 – 700 km, the African LLSVP reaches a height of ~1,500 – 1,800 km from the core-mantle boundary. With numerical modeling experiments, we find that the height of the Pacific LLSVP can be reconciled by a wide range of the LLSVP density and surrounding mantle viscosity. However, the large height of the African LLSVP indicates that it may be less dense and thus less stable than the Pacific LLSVP.
The Kuluketage block is the best area for Precambrian geology in north western China, because it contains the most complete Precambrian lithology units. Thus, the study of this ancient basement can improve the understanding of the Precambrian evolution of the Tarim Craton. In this study, we report LA-ICPMS zircon U-Pb ages and Hf isotopes of detrital zircons from a magnetite quartzite from the Shayiti Formation of the Xingditage Group. The 65 zircon ages and Hf isotopes obtained are used not only to constrain the maximum depositional ages of the Shayiti Formation but also to obtain the information about the evolution of regional tectonic-magmatic activities in the Paleoproterozoic of the Kuluketage block. According to the youngest concord 207Pb/206Pb zircon age of 1851 ± 36 Ma in magnetite quartzites and the 1.47 Ga of the diabase sills which intrude into the Shayiti Formation, the most probable depositional age of the Shayiti Formation is between 1.47 Ga and 1.85 Ga. The detrital zircon dates are mainly clustered at 1806 Ma to 1889 Ma, 1898 Ma to 1981 Ma, and 1988 Ma to 2054 Ma, with the most prominent age peak appearing at around 1900 Ma and the subordinate peak age at around 1960 Ma. The magmatic features of Cathodoluminescence (CL) images indicate that two large magmatic tectonic-magmatic activities occurred in this district. The metamorphic rims of magmatic zircons and some baddeleyites also show regional metamorphism in the Paleoproterozoic, which may be related to the amalgamation of the Columbia supercontinent. We obtained two sets of concordant U-Pb ages older than 2.5 Ga, and several sets of two-stage Hf model ages older than 3.0 Ga. Combined with previous data in the literature, we suggest that Meso- to Neo-Archean basement rocks existed in the Kuluketage block, but were strongly reformed by tectonics, magmatism, and metamorphism in the Paleoproterozoic.
Abstract The Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon 206 Pb/ 238 U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO 2 , Zr(Nb) and Al 2 O 3 and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al 2 O 3 contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.
During the past 50 years, many geological and ore-deposit investigations have led to the discovery of the Fe–P–(Ti)-oxide deposits associated with mafic–ultramafic–carbonatite complexes in the Kuluketage block, northeastern Tarim Craton. In this paper, we discuss the genetic and ore-forming ages, tectonic setting, and the genesis of these deposits (Kawuliuke, Qieganbulake and Duosike). LA-ICP-MS zircon U–Pb dating yielded a weighted mean 206Pb/238U ages of 811 ± 5 Ma, 811 ± 4 Ma, and 840 ± 5 Ma for Kawuliuke ore-bearing pyroxenite, Qieganbulake gabbro and Duosike ore-bearing pyroxenite, respectively. The CL images of the Kawuliuke apatite grains show core–rim structure, suggesting multi-phase crystallisation, whereas the apatite grains from Qieganbulake and Dusike deposits do not show any core–rim texture, suggesting a single-stage crystallisation. LA-ICP-MS apatite 207Pb-corrected U–Pb dating provided weighted mean 206Pb/238U ages of 814 ± 21 Ma and 771 ± 8 Ma for the Kawuliuke ores, and 810 ± 7 Ma and 841 ± 7 Ma for Qieganbulake and Duosike ores, respectively. The core–rim texture in apatite by CL imaging as well as two different ore-forming ages in the core and rim of the apatite indicate two metallogenic events for the Kawuliuke deposit. The first metallogenic period was magmatic in origin, and the second period was hydrothermal in origin. The initial ore-forming age of the Kawuliuke Fe–P–Ti mineralisation was ca 814 Ma and the second one was ca 771 Ma. On the other hand, the ore-forming ages of the Qieganbulake and Duosike deposits were ca 810 Ma and ca 841 Ma, respectively. Qieganbulake and Duosike deposits were of magmatic origin. Combined with previous geochronological data and the research on the tectonic background, we infer that the Kawuliuke, Qieganbulake and Duosike Fe–P–(Ti)-oxide deposits were formed in a subduction-related tectonic setting and were the product of subduction-related magmatism.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
