When modern-style plate tectonics started and operated on a specific old craton has been a hot topic involving the early Earth's evolution. In order to address this issue on the evolution of the Neoarchean North China Craton (NCC), we investigated a newly identified successive magmatic rock suite of tonalite-trondhjemite-granodiorite (TTG)-sanukitoid in the Datong-Huai’an Complex. Geochemically, TTG gneisses in the Datong-Huai’an Complex can be divided into high-pressure (HP) and low-pressure (LP) TTG rocks. The HP TTGs are characterized by steep rare earth element (REE) patterns, obvious negative Nb, Ta and Ti anomalies and positive δEu anomalies, high Sr/Y, Nb/Ta and (La/Yb)N ratios, and positive Hf (t) (+2.1 to +8.7) and δ18O values (Ave 5.5‰ to 6.0‰). Their protolith is interpreted as the consequence of partial melting of a subducted oceanic slab with garnet and minor rutile as residual phases. The LP TTGs feature flat REE patterns, slightly negative Nb, Ta and Ti anomalies and negative δEu anomalies, low Sr/Y, Nb/Ta and (La/Yb)N ratios, positive Hf (t) (+3.4 to +5.9) and high δ18O values (Ave 5.9‰ to 6.1‰). They may have been generated by partial melting of mafic lower crust with residual plagioclase and amphibolite in the source. The sanukitoid rocks show high MgO, Cr and Ni concentrations but relatively low (La/Yb)N values, positive ɛHf (t) values (+1.5 to +5.4), and higher δ18O values (Ave 1.9‰ to 8.7‰), suggesting that they were originated from partial melting of mantle peridotite previously modified by slab-derived or sediment-derived melts. Zircon U–Pb dating results reveal that the HP TTGs formed at ~2538 Ma and ~2479–2441 Ma, whereas the LP TTGs and sanukitoids formed at ~2518 Ma and ~2517–2485 Ma, respectively. Combining rock assemblages, geochemical features and geological data, we propose that this long-lived magmatism can be divided into three stages of ~2.55-2.52 Ga, ~2.52-2.48 Ga and ~2.48-2.44 Ga. The Datong-Huai’an Complex may have developed along an active continental margin and depicts modern-style plate tectonics with continuous steep subduction, slab rollback and back-arc extension in the late Neoarchean.
Abstract Deciphering the formation and geodynamic evolution of high-pressure (HP) granulites in a collisional orogeny can provide crucial constraints on the geodynamic evolution of subduction-exhumation. To fully exploit the geodynamic potential of metamorphic rocks, it is necessary to constrain the metamorphic ages, although it is difficult to link zircon and monazite ages to metamorphic evolution. A good case study for understanding these geodynamic processes is felsic granulites in the Bashiwake area, South Altyn Tagh. Petrographic observations suggest that the studied felsic granulites have suffered multi-stage metamorphism, and the distinct metamorphic events were documented by compositional zoning and high Y + heavy rare earth element (HREE) concentrations in the large garnet porphyroblast. Zircon U-Pb dating yielded two major age clusters: one age cluster at ca. 900 Ma represents the age of the protolith for the felsic granulite, and another age cluster at ca. 500 Ma represents the post-UHT (ultrahigh temperature) stage based on the rare earth element distribution coefficients between zircon and garnet. Meanwhile, in situ monazites U-Pb dating yielded a weighted mean 206Pb/238U age of 482 ± 3.5 Ma, and the monazite U-Pb age was interpreted to be in agreement with the metamorphic zircon rims data, which together with zircon recorded the cooling time after the UHT stage. Whole-rock major and trace elements as well as Sr-Nd isotopes suggest that the protolith of the felsic granulite derived from partial melting of ancient crustal materials with the addition of mantle materials. Integrating these results along with previous studies, we propose that the felsic granulites metamorphosed from the Neoproterozoic granitic rocks, and the granitic rocks with associated mafic-ultramafic rocks suffered a common high-pressure–ultrahigh temperature (HP-UHT) metamorphism and subsequent granulite-facies metamorphism. A tentative model of subduction-relamination was proposed for the geodynamic evolution of the Bashiwake unit, South Altyn Tagh.
A network of four SADAR® arrays installed at Carbon Management Canada’s (CMC) Containment and Monitoring Institute (CaMI) Field Research Station provides an example of the results achievable through passive monitoring of microseismicity at an active CO2 storage facility. The SADAR arrays, designed as compact volumetric phased arrays, provide a passive, persistent, and permanent data acquisition and analysis capability. Data from compact phased arrays are processed to take advantage of the spatial coherence of the incident seismic signals to increase signal resolution while suppressing noise and clutter signals, and simultaneously providing signal attributes such as angle-of-incidence and phase velocity. The network of arrays allows for automation of location and magnitude determination at a reduced channel count and sensor footprint. We present results from a nine-day reporting period, a subset of the overall compiled seismic event bulletin, chosen because the time span contains both CO2 injection events as well as other non-injection activities. A total of 55 events were detected and located with an Mw = -2.5 threshold. The results demonstrate the promising performance of permanently deployed, networked SADAR arrays to detect and locate microseismicity associated with CO2 storage reservoirs. Technologies such as SADAR will be an enabling driver as industries embark upon gigatonne storage capacities.
The Altyn Tagh,located on the southeastern margin of the Tarim basin,is a main region where metamorphic basement of the Tarim craton is exposed.In this paper,zircon U-Pb isotopic data of sedimentary rocks unconformably overlying on Archaean-Early Paleoproterozoic metamorphic basement and high grade metamorphic rocks from the Cental-South Altyn Tagh are used to investigate the nature and ployphase tectonothermal events of the metamorphic basement in the southeastern margin of the Tarim basin.Detrital zircons from Neoproterozoic Annanba Group overlying on the Tarim basement yield mainly an age at ca.1.92Ga,with a few grains between 2.0Ga and 2.4Ga,suggesting that detrital material of the Annanban Group shed from the underlying Milan Group and associated plutonic rocks.In the Central Altyn Tagh massif and South Altyn Tagh subduction-collision complex belt,zircon U-Pb chronological data show that high grade metamorphic rocks recorded three phase tectonothermal events: Early Neoproterozoic event (900~940Ma),Late Neoproterozoic event (760Ma) and Early Paleozoic event.Early Neoproterozoic tectonothermal event can be linked to the Tarim (or Jinning) orogeny,which was commonly recognized in the metamorphic basement around the Tarim basin and the South China block (or Yangtze craton),and was related to the formation of the Rodinia supercontinent.Late Neoproterozoic tectonothermal event has been also commonly recognized around the Tarim basin and Yangtze craton and was related to the breakup of Rodinia supercontinent.Therefore,in Neoproterozoic era,the Altyn Tagh has a similar evolution history to the Yangtze craton,differing from the North China craton.Zircon U-Pb chronological data also indicate that the high grade metamorphic rocks from the Central-South Altyn Tagh commonly experienced Early Paleozoic metamorphic overprint related to subduction and collision.
Generally the quantitative interpretation of the fault uses a 2D model. But if the depth and extension of the fault increase,the inversion results will have a bigger error because the 2d model is not fit for the real fault.In this paper,we put forward a 2.5D model based on the hexahedron and developed an inversion method of constraint gravity-vertical fault.The inversion result is better than that the 2D model through the model tests.Based on this model,we quantitatively interpreted the Bouguer gravity anomaly of the Sierra Nevada Region in the USA.
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