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3 Geotourism / Geoturystyka

3 2023

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Temporal and spatial heterogeneity of the Ailaoshan–Song Ma–Song Chay ophiolitic mélange, and its significance on the evolution of Paleo-Tethys

Lin Wei, Liu Fei, Wang Ying, Meng Lingtong, Faure Michel, Chu Yang, Nguyen Vuong Van, Wu Qinying, We Wei, Thu Hoai Luong Thi, Vu Tich Van

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

42--43

The ophiolite is the direct evidence to restore the oceanic evolution, and it is used to identify the convergence boundary of the plates. Compared with ophiolite, ophiolitic mélange, especially its matrix, contains more information about the evolution of ocean. The evolution of eastern Paleo-Tethys, between the South China and Indochina blocks, recorded the whole process of rifting from Gondwana and their northward migration and convergence. To understand the tectonic implications from matrix of ophiolitic mélange, the Mesozoic Paleo-Tethys Ailaoshan–Song Ma–Song Chay suture zone located in the North Vietnam–Southeast Yunnan region acts as an ideal study area. Based on the structural geology, we reviewed previous zircon U-Pb dating and Lu-Hf isotopic analyses on the detrital zircon from the Ailaoshan–Song Ma–Song Chay ophiolitic mélange. Accordingly, we subdivide the matrix of these ophiolitic mélange into four parts (M1, M2, M3, and M4; Fig. 1). M1 is mainly located in the middle segment of the Ailaoshan–Song Ma belt. It shows age peaks of 440 Ma and 960 Ma with εHf(t) values of −19.6 ~ +10.3. M2 is mainly located in the NW segment of the Ailaoshan–Song Ma belt, showing a dominant age peak of ~260 Ma. Particularly, it has εHf(t) values of −28.9 ~ +8.1. M3 is mainly located in the SE segment of the Ailaoshan–Song Ma belt, showing the peaks at ~250 Ma, 440 Ma, and 960 Ma with εHf(t) values of −21.9 ~ +10.1. M4 is mainly located in the Song Chay belt, showing the peaks at ~310 Ma, 470 Ma, 610 Ma, 770 Ma, and 965 Ma with εHf(t) values of −28.2 ~ +10.8. The geochronological data of the detrital zircon from the matrix of the Ailaoshan– Song Ma–Song Chay ophiolitic mélange zone, documents a temporal heterogeneity between the M1, M2, M3, and M4 units, which formed at 310–270 Ma, 265–250 Ma, 245–240 Ma, and 310–255 Ma, respectively. The different components and provenances of each unit reflect a strike-parallel heterogeneity (Fig. 1). The M1 unit was mainly sourced from the Paleozoic sedimentary rocks of the Indochina Block (IB). The main provenance for the M2 unit is Emeishan Large Igneous Province (ELIP). The magmatic arc developed in the IB provided the materials for the M3 unit, and the detrital materials of the M4 were mainly sourced from the South China Block (SCB) (Fig. 1). The Cenozoic strike-slip deformation led to an inverted geometry of the M1, M2, and M3 units, accounting for a strike-perpendicular heterogeneity straight to the strike of the orogenic belt. The temporal, strike-parallel, and strike-perpendicular heterogeneity help us to decipher the tempo-spatial evolution of the Paleo-Tethys. The M1, M2, M3, and M4 units contain information from different evolutionary stages, likely recording the comprehensive history of the ancient oceanic basin. Importantly, our results demonstrate that both the active continental margin of the IB and the passive continental margin of the SCB acted as provenance sources that supplied significant amount of detrital material in the ophiolitic mélange matrix, indicating that the Paleo-Tethys Ocean was a “narrow” or “limited” ocean rather than the archipelagic ocean proposed before.

Coevolution of Paleo-Tethys and Rheic: New tectonic constraints from Iran and Turkey

Chu Yang, Wan Bo, Lin Wei, Allen Mark B., Talebian Morteza, Uysal Ibrahim, Xin Guangyao

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

15--15

In the Paleozoic, one large ocean once separated the Eurasia of the north and the Gondwana of the south, but it has two names, Paleo-Tethys and Rheic, suggesting different tectonic history. The Paleo-Tethys represent the ocean from east Asia to Middle East regions and vanished in Early Mesozoic, while the Rheic existed across the Europe and finally closed in Carboniferous. The two oceans coevolved for a long time, but the interaction and mutual effect at subduction and collision stages are not well understood. Initiation processes of ocean spreading, subduction and collision are crucial in plate tectonics, so resolving the timing for these turning points may greatly enhanced the precision and accuracy of reconstruction of the two oceans, especially for the western Paleo-Tethys. In NE Iran, we find that all the Paleozoic clastic rocks record two major zircon U-Pb age groups peaked at ~800 Ma and ~600 Ma. Consistency in age patterns show a dominant provenance from Neoproterozoic basement of the north Gondwana and a long-lasting passive margin sedimentation after the spreading of the Paleo-Tethys. This environment was interrupted by initial collision between the Turan (Eurasia) and Central Iran (Gondwana) Blocks with massive coarse clastic deposition, i.e. the protolith of the Mashhad Phyllite, in a peripheral foreland basin on the Paleozoic passive margin. The Mashhad Phyllite yields a striking provenance change from passive margin to active margin. The Paleozoic ages reveal a long-lived subduction zone at the south Turan Block initiated since the latest Ordovician. More importantly, the provenance shift better constrains the initial collision timing with the maximum deposition age of the Mashhad Phyllite (~228 Ma) refining the evolution history of Paleo-Tethys. Based on our new results and previous data, we compare the tectonic history of the Paleo-Tethys in its western segment with eastern Rheic, and further discuss the interaction between the Rheic and Paleo-Tethys. We find existence of a lateral subduction zone plays a crucial rule in initiating new subduction zone after an old oceanic plate vanishes and two continents collides, while a lateral collision can also result into shallowing of subducted slab and preservation of coeval compressional structures. These new insights help us to better interpret the emplacement of high-pressure metamorphic rocks during subduction and subduction zone jump when the Rheic and Paleo-Tethys coevolved.

Bulldoze and rebuild: Modifying cratonic lithosphere via removal and replacement induced by continental subduction

Meng Lingtong, Chu Yang, Lin Wei, Mitchell Ross N., Zhao Liang

Geotourism / Geoturystyka

2023

nr 1-2 (72-73)

47--48

Establishing the mechanisms for craton modification is critical for understanding cratonic stability and architecture. Both plate tectonics and mantle plumes can cause weakening, mechanical decoupling, and even lithospheric removal. But craton modification – craton destruction accompanied or followed by craton rejuvenation – has received less attention. It is well-known that oceanic subduction dominantly destroys cratonic lithosphere with replacement to a lesser degree, and mantle plumes have been related to both destruction and rejuvenation. The role of continental subduction in craton modification, however, remains a comparatively open question. The North China Craton, as a previously stable continent with a lithosphere of more than 200 km since the Paleoproterozoic, was reworked and substantially destroyed since the Mesozoic, with intensive destruction occurring in the Early Cretaceous. Earlier in the Mesozoic, North China Craton experienced a continent-continent collision (as the upper plate) with the South China Block, forming the Sulu orogenic belt, providing an opportunity to understand the potential for craton modification due to deep continental subduction In the North China craton, we report the presence of material (i.e., Yunshan unit) sourced from the underlying subducted plate. It is composed of foliated monzonitic granite and metamorphic sedimentary rocks that locally experienced crustal anatexis. Through detailed zircon U-Pb dating, it formed at latest Triassic (ca. 212 Ma). Importantly, the 800–700 Ma inherited zircons from the Yunshan foliated granite resemble those from the South China Block rather than the North China Craton. According to structural and magnetic data, the fabrics of the Yunshan foliated granite, characterized by gentle magnetic/mesoscopic foliations and conspicuous NW-SE-trending magnetic/mesoscopic lineations with a top-to-the-NW shearing. Its geometry, kinematics, and timing all compare favorably with the latest Triassic extensional structure accounting for the exhumation of the Sulu orogenic belt. We thus interpret the Yunshan unit to have been sourced from the subducted South China Block, then exhumed and emplaced into the overriding North China Craton (Fig. 1A). Combining our new results with previous geological and geophysical data, we argue that from 250–220 Ma a 200-km-long tract of North China Craton lithosphere was bulldozed by the subducted South China Block, resulting in a lithospheric suture far from the suture zone at the surface. This lithospheric removal occurred at mid-lower crustal levels (16–20 km depth) – much shallower than previously thought possible. The bulldozed North China Craton lithosphere was simultaneously replaced by the reworked underlying South China Block plate. Such a “bulldoze and rebuild” lithospheric modification process minimized asthenosphere-lithosphere interaction, thus preventing the North China Craton from further modification (Fig. 1B–1D). Because there was essentially no net loss of lithosphere during deep continental subduction, the North China Craton largely maintained its stability for the time and did not suffer intensive destruction until later Early Cretaceous palaeo-Pacific oceanic subduction. This “bulldoze and rebuild” model can thus account for how a craton can maintain its stability during a collision with another continental plate.