The Xiaobaishitou gabbro-diorite pluton comprises a medium-grained gabbro-diorite suite and a fine-grained diorite suite, which intrude the Kawabulag Group in the East Tianshan Orogen of the Central Asian Orogenic Belt(CAOB). A combination of mineral chemistry, zircon U-Pb age, whole-rock geochemistry, Sr-Nd isotopes, and in situ zircon Hf isotopes for newly found gabbro-diorite from the Xiaobaishitou district in the Central Tianshan Terrane(CTT) is presented to investigate the petrogenesis and ... 相似文献
AbstractThe turbidite-filled, Lower Devonian Cobar Basin is characterised through a detrital zircon study. Uranium–Pb age data for six samples were combined with published data to show the basin has a unique age spectrum characterised by a subordinate Middle Ordovician (ca 470?Ma) peak superimposed on a dominant ca 500?Ma peak. Maximum depositional ages for 3 samples were ca 425?Ma, close to the published Lower Devonian (Lochkovian 419–411?Ma) biostratigraphic ages. A minor ca 1000?Ma zircon population was also identified. The major source of the 500?Ma zircons was probably the local Ordovician metasedimentary basement, which was folded, thickened and presumably exposed during the ca 440?Ma Benambran Orogeny. The ca 470?Ma age peak reflects derivation from Middle Ordovician (Phase 2) rocks of the Macquarie Arc to the east. The I-type Florida Volcanics, located ~50?km eastward from the Cobar Basin, contains distinctive Middle and Late Ordovician zircon populations, considered to be derived from deeply underthrust Macquarie Arc crust. Protracted silicic magmatism occurred before, during and after Cobar Basin deposition, indicating that the basin formed by subduction-related processes in a back-arc setting, rather than as a continental rift. 相似文献
AbstractFour oroclinal structures have been identified from structural, magnetic and gravity trends across a Carboniferous continental arc, forearc basin [Tamworth Belt (TB)] and conjugate accretionary complex in the southern New England Orogen (SNEO) of eastern Australia. None of the structures has yet been confirmed conclusively by paleomagnetism as oroclinal. Ignimbrites are common within the forearc basin and have been demonstrated to retain primary magnetisations despite prevalent overprinting. They are well exposed across six major tectono-stratigraphic blocks with partly interlinked stratigraphies, making the forearc basin highly prospective to oroclinal testing by comparing pole path segments for individual blocks across curved structures. Paleomagnetic studies have shown no noticeable rotation across the western/southwestern TB (Rocky Creek, Werrie and Rouchel blocks), but documented herein is a minor counter-clockwise rotation of the Gresford Block of the southern TB. This study details paleomagnetic, rock magnetic and magnetic fabric results for 87 sites (969 samples) across the southern Gresford Block. Predominantly thermal, also alternating field and liquid nitrogen, demagnetisations show a widely present low-temperature overprint, attributed to regional late Oligocene weathering, and high-temperature primary and overprint components residing in both mainly magnetite and mainly hematite carriers. Subtle, but systematic, directional differences between magnetite and hematite subcomponents show the latter as the better cleaned, better-defined, preferred results, detailing nine primary poles of middle and late Carboniferous ages and Permian and Permo-Triassic overprints as observed elsewhere in the western/southwestern TB. The primary poles update a poorly defined mid-Carboniferous section of the SNEO pole path and demonstrate counter-clockwise rotation, quantified at about 15° ± 13° from comparison of mid-Carboniferous Martins Creek Ignimbrite Member poles for the Rouchel and Gresford blocks, that may not necessarily have been completed prior to the Hunter–Bowen phase of the Gondwanide Orogeny. This minor counter-clockwise rotation of the Gresford Block accentuates a primary curvature of the southwestern/southern TB and heralds further, more complex, rotations of the Myall Block of the southeastern TB. 相似文献
AbstractThis paper summarises current knowledge on metamorphism within the entire New England Orogen (NEO) of eastern Australia. Rocks recording metamorphic assemblages characteristic of each of the three metamorphic facies series (high, medium and low P/T) have been identified within the orogen. These include high P/T blueschists and eclogites, mid P/T orogenic metamorphism and low P/T contact aureoles and sub-regional high-temperature–low-pressure (HTLP) metamorphism (regional aureoles). Metamorphism is described as it relates to six tectonic phases of development of the NEO that together comprise two major cycles of compression–extension. Medium–high-grade contact metamorphism spans all six tectonic phases while low-grade burial and/or orogenic metamorphism has been identified for four of the six phases. In contrast, exposure of high P/T eclogites and blueschists, and generation of sub-regional low P/T metamorphism is restricted to extensional phases of the orogen. Hallmarks of the orogen are two newly identified zones of HTLP metamorphism, the older of which extends for almost the entire length of the orogen.
KEY POINTS
The orogen is dominated by low-temperature rocks while high-temperature amphibolite to granulite facies rocks are restricted to small exposures in HTLP complexes and contact aureoles.
Blueschist metamorphism falls into two categories; that associated with subduction during the Currabubula-Connors continental arc phase occurring at depths of ~13–30?km; and the other of Cambrian–Ordovician age, exposed within a serpentinite melange and associated with blocks of eclogite. The eclogite, initially from depths of ~75–90?km, appears to have been entrained in the deep crust for an extended period of geological time.
A comprehensive review of contact metamorphism in the orogen is lacking and as studies on low-grade metamorphism are more extensive in the southern part of the orogen than the north, this highlights a second research gap.
This paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr-Nd and zircon Hf-O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U-Pb dating of samples in the three areas yielded weighted mean 206 Pb/238 U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 828.3 ± 8.5 Ma and 836.9 ± 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO2(54.08–76.80 wt%) and Al2 O3(12.40–21.31 wt%), low Fe2 O3(0.68–8.92 wt%), MgO(0.29–2.49 wt%), CaO(0.12–2.86 wt%), TiO2(0.10–1.59 wt%) and P2 O5(0.01–0.39 wt%), with variable total alkalis(K2 O + Na2 O =5.42–8.29 wt%). There exists a clear negative correlation between SiO2 and P2 O5. The volcanic rocks have A/CNK ratios of 1.03–2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of(La/Yb)N values of 5.68–11.67, and with a moderate negative Eu anomaly(Eu=0.58–0.89). Whole-rock geochemical data shows that the Jiangshan volcanic rocks are possibly I-type granitic rocks, even though they have some characteristics of AS-type granites due to the magma fractional crystallization and water-rock interaction. Zircon δ18 O values are 3.97‰–5.49‰(average 4.50‰), 2.90‰–5.21‰(average 4.32‰) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average δ18 O values of igneous zircons in equilibrium with mantle magmas(5.3 ± 0.6‰(2σ)), the lower δ18 O value also demonstrating the presence of high temperature water-rock interactions. The ignimbrite rocks have positive εNd(t)(4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and εNd(t)(in-situ zircon)(4.3–14.6, a weighted mean of 8.4; 6.6–12.7, a weighted mean of 9.0; 8.1–12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle-sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860–830 Ma volcanic rocks formed in a back-arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan–Shaoxing suture. The first stage rift-related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen. 相似文献