Episodic Growth of the Continental Crust in SE China: Nd Isotopic Approach

Tonalite, Trondhjemite, Granodiorite (TTG) rocks in Viti Levu, Fiji islands formed through hydrous melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek. During mid Miocene time, magmatic underplating took place and a Qtz-diorite unit was formed out of the gabbro under granulite-facies conditions. The investigated TTG&#180;s occur as stocks and veins within the older gabbroic unit of the Yavuna Pluton.Zircon ages show the parental gabbro to be ~47.5 Ma in age, whereas the TTG&#180;s, which can be subdivided into a tonalite and a Qtz-diorite suite, are ~37.1 Ma and ~16.5 Ma, old respectively. The average d18O value of ~4.8 in zircon selected from the parental gabbro and the tonalite suggest a very homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples showing lower d18O values, and these are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep depleted mantle source. eHf&#61512;&#61542; in zircon values of ~13 in the analyzed TTG&#180;s are interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations suggest that the tonalite formed by melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ~770 &#176;C and a pressure of ~3.8 kbar, whereas the Qtz-diorite formed at a temperature up to ~900 &#176;C at very shallow depth close to the Earth&#8217;s surface caused by the emplacement of a magmatic underplate during the mid Miocene. Our investigation provides new evidence for episodic growth of continental crust < 0.1 Ga in the South Pacific region.

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Structural Evolution of Extended Continental Crust Deciphered From the Cretaceous Batholith in SE China, a Kinmen Island Perspective

Frontiers in Earth Science ◽

10.3389/feart.2020.00330 ◽

2020 ◽

Vol 8 ◽

Author(s):

Tsung-Han Huang ◽

Meng Wan Yeh

Keyword(s):

Continental Crust ◽

Structural Evolution ◽

Se China

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Tectono-magmatic and thermal evolution of the SE China margin-NW Palawan breakup

10.5194/egusphere-egu2020-14296 ◽

2020 ◽

Author(s):

Geoffroy Mohn ◽

Michael Nirrengarten ◽

Andrea Schito ◽

Nick Kusznir ◽

Sveva Corrado ◽

...

Keyword(s):

Continental Crust ◽

Thermal Evolution ◽

Oceanic Lithosphere ◽

Seafloor Spreading ◽

Spreading Ridge ◽

Thermal Maturity ◽

Seismic Profiles ◽

Thinned Continental Crust ◽

Conjugate Margins ◽

Se China

Continent Ocean Transitions (COTs) record the processes leading to continental breakup and localized oceanic accretion initiation. The recent IODP Expeditions 367-368 and 368X at the SE China margins combined with high quality multi-channel seismic profiles provide a unique dataset to explore the tectono-magmatic and thermal evolution from final rifting to early seafloor spreading. To investigate these issues, we developed a multi-disciplinary approach combining reflection seismic interpretations with geophysical quantitative analysis calibrated thanks to drilling results, from which we measured and modelled the thermal maturity in pre-/syn- to post-rift sediments.Drilling results show that the transition from the most thinned continental crust to new, largely igneous crust is narrow (~20 km). During final rifting, decompression melting forming Mid-Ocean Ridge type magmatism emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks concomitant with continued deformation by extensional faults. The initial igneous crust of the conjugate margins is asymmetric in width and morphology. The wider and faulted newly accreted domain on the SE China side indicates that magmatic accretion was associated with tectonic faulting during the formation of initial oceanic lithosphere, a feature not observed on the conjugate Palawan side. We suggest that deformation and magmatism were not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically prior to the spreading ridge stabilising.Organic matter from post-rift sediments has low thermal maturities due to limited burial and the absence of late post-rift magmatism. In contrast, pre to syn-rift sediments show significant variability in thermal maturities across the COT. Localised high thermal maturities for the pre- to syn-rift sediments requires that significant additional heat be imparted at shallow depths during breakup, likely related to magmatic intrusion or subsurface expressions of volcanism. The heterogeneous variation in thermal maturity observed across the COT reflects localised thermal perturbations caused by magmatic additions.

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