Development of an intra-carbonate detachment during thrusting: The variable influence of pressure solution on deformation style, Khao Khwang Fold and Thrust Belt, Thailand

Classic detachment zones in fold and thrust belts are generally defined by a weak lithology (typically salt or shale), often accompanied by high over-pressures. This study describes an atypical detachment that occurs entirely within a relatively strong Permian carbonate lithology, deformed during the Triassic Indosinian orogeny in Thailand under late diagenetic-anchimetamorphic conditions. The key differences between stratigraphic members that led to development of a detachment zone are bedding spacing and clay content. The lower, older, unit is the Khao Yai Member (KYM), which is a dark-gray to black, well-bedded, clay-rich limestone. The upper unit, the Na Phra Lan Member (NPM), comprises more massive, medium- to light-gray, commonly recrystallized limestones and marble. The KYM displays much tighter to even isoclinal, shorter-wavelength folds than the NPM. Pressure solution played a dominant role throughout the structural development—first forming early diagenetic bedding; later tectonic pressure solution preferentially followed this bedding instead of forming axial planar cleavage. The detachment zone between the two members is transitional over tens of meters. Moving up-section, tight to isoclinal folds with steeply inclined axial surfaces are replaced by folds with low-angle axial planes, thrusts, and thrust wedging, bed-parallel shearing, and by pressure solution along bedding-parallel seams (that reduce fold amplitude). In outcrops 100–300 m long, reduction of line-length shortening on folds from >50% to <10% shortening upwards indicates that deformation in the NPM is being accommodated differently from the KYM, probably predominantly by shortening on longer wavelength and/or spacing folds and thrusts, given the low amount of strain observed within the NPM, which excludes widespread layer-parallel thickening

Petrogenesis and Geochronology of Tianshui Granites from Western Qinling Orogen, Central China: Implications for Caledonian and Indosinian Orogenies on the Asian Plate

The precise timing, petrogenesis, and geodynamic significance of three granitoid bodies (Beidao granite, Caochuanpu granite, Yuanlongzhen granite, and the Roche type rock) of the Tianshui area in the Western Qinling Orogen, central China, are poorly constrained. We performed an integrated study of petrology, geochemistry, and zircon U-Pb dating to constrain their genesis and tectonic implication. Petrographic investigation of the granites shows that the rocks are mainly monzogranites. The Al saturation index (A/CNK versus SiO2) of the granitoid samples indicates meta-aluminous to peraluminous I-type granites. Their magmas were likely generated by the partial melting of igneous protoliths during the syn-collisional tectonic regime. Rare-earth-elements data further support their origin from a magma that was formed by the partial melting of lower continental crust. The Beidao, Caochuanpu, and Yuanlongzhen granites yielded U-Pb zircon weighted mean ages of 417 ± 5 Ma, 216 ± 3 Ma, and 219 ± 3 Ma, respectively. This study shows that the Beidao granite possibly formed in syn- to post-collision tectonic settings due to the subduction of the Proto-Tethys under the North China Block, and can be linked to the generally reported Caledonian orogeny (440–400 Ma) in the western segment of the North Qinling belt, whereas Yuanlongzhen and Caochuanpu granites can be linked to the widely known Indosinian orogeny (255–210 Ma). These granitoids formed due to the subduction of the oceanic lithospheres of the Proto-Tethyan Qinling and Paleo-Tethyan Qinling. The Roche type rock, tourmaline-rich, was possibly formed from the hydrothermal fluids as indicated by the higher concentrations of boron leftover during the late-stages of magmatic crystallization of the granites.

Discovery of Middle–Late Devonian and Early Permian magmatic events in East Asia and their implication for the Indosinian orogeny in South China: Insights from the sedimentary record

Whether the driver of the Indosinian orogeny in the South China block was related to the evolution of the Paleotethyan Ocean or the Paleo-Pacific Ocean has been a point of much debate. We applied detrital zircon U-Pb dating to Permian–Triassic sedimentary rocks from South China to trace sediment provenance and to further test these models. Our results, combined with other published data from the Pingxiang, Youjiang, Yong’an, and Yongding Basins, show that 400–350 Ma and 300–260 Ma zircon grains are ubiquitous throughout the entirety of southern South China. This indicates regional magmatic events as potential sources. The discovery of Middle–Late Devonian and Early Permian igneous rocks, tuffs, and volcaniclastic rocks in Southeast Asia and Hainan Island implies the presence of two magmatic events (400–350 Ma and 300–260 Ma) within or beyond the southern margin of South China. This information, together with the mostly negative εHf(t) values of 400–350 Ma and 300–260 Ma zircon grains, arc-like geochemical signatures of the possible source rocks, and the regional geology of East Asia, suggests that they originated from sources related to Paleotethyan and even Proto-Tethyan subduction. Thus, Permian–Triassic sedimentation and the Indosinian orogeny in South China were largely controlled by the evolution of the Tethyan Ocean.

The appearance of copper-bearing mineralization in the region of Muang Xay, Laos

Geotectonic events in the Indochina Peninsula, associatted with the Indosinian orogeny between the Paleozoic and Mesozoic, has created many fold structures and fault areas oin the territory of Laos. Magmatic processes and Earth’s crust deformation enabled arising of rich Cu mineralization. A prospective area for the occurrence of Cu, Ag and Au minerals is the folded region on Muang Xay, northern Laos. The main purpose of geological research of the Muang Xay area was the recognition of chemical and mineralogical composition of rock samples and the evaluation of the area of potential geological resources. During field reconnaissance in 2018, the rock samples were taken from an abandoned mine. It was concluded that primary rocks suffered intense Na-metasomatic processes, preserving primary copper sulphides – bornite, chalcocite and covellite. The content of Cu is up to 49.64% and Ag up to 119.5 ppm. As a result of metasomatism of igneous rocks, diorite or andesite types – albitite was formed. Albitite is composed of medium-grained albite, Mg-chlorite and Cu-carbonates. The prospective research shows that the Muang Xay region has a high potential of metalliferous deposits.