Indian slab lurches downward beneath Afghanistan

As I walked into the department this bright brisk morning, coffee cheerily in hand, the live global seismogram display in the atrium caught my eye with an alarming event that had just happened during my bike ride into work. BIG earthquake, somewhere in the vicinity of Central/Southern Asia. Indeed, an earthquake deep (>200 km) beneath the Hindu Kush mountains of Afghanistan had shaken a huge swath of Central and South …

Tectonic setting of Triassic magmatic and metallogenic event in the Duobaoshan mineralization area of Heilongjiang Province, NE China

The Duobaoshan mineralization area is located in the northeast part of the Xing’an–Mongolia orogenic belt which belongs to the eastern part of the Central Asian Orogenic Belt (CAOB). This region has experienced multiple tectonic–magmatic–metallogenic events that resulted in a number of large–superlarge mineral deposits, e.g. the early Palaeozoic Duobaoshan and Tongshan porphyry Cu–Mo deposits, the early Mesozoic Sankuanggou skarn Fe–Cu deposit and Xiaoduobaoshan skarn Cu deposit and the late Mesozoic Zhengguang epithermal Au deposit. By detailed field survey, we have examined the weak alteration and mineralization in the tonalite veins intruding early Palaeozoic ore‐bearing granodiorite in the Duobaoshan deposit and the veinlet‐disseminated Cu–Mo mineralization in the porphyritic granite veins intruding in the ore‐hosting Ordovician strata of the Duobaoshan Formation in the Tongshan deposit. Zircon grains from the Duobaoshan tonalite, the Tongshan porphyritic granite, and the metallogenic granite porphyry and the granodiorite in the Xiaoduobaoshan deposit yield LA–ICP–MS U–Pb weighted mean ages of 230.9 ± 2.3 Ma, 235.4 ± 2.7 Ma, 226.6 ± 1.7 Ma and 232.8 ± 1.7 Ma, respectively. In addition, the Re–Os model age of molybdenites from the Tongshan porphyritic granite is 229.4 ± 3.5 Ma. Those isotope dating results indicate that the Duobaoshan mineralization area experienced the Triassic magmatic–mineralization event. Considering the regional tectonic history of the area and the fact that the major and trace element characteristics of the Triassic tonalite in the Duobaoshan deposit are similar to those of island–arc type igneous rocks, it is presented that the Triassic magmatic–metallogenic event might be closely related to the subduction of the Mongolia–Okhotsk plate, and that the metallogenic elements originated from the mantle wedge metasomatized by liquid of subduction oceanic slab. Previous research and new geochemical analyses presented in this study indicate that the Duobaoshan mineralization area experienced at least four epochs of magmatic–metallogenic events including the Ordovician, the Triassic, the Early Jurassic and the Late Jurassic to Early Cretaceous. Copyright © 2015 John Wiley & Sons, Ltd.

Deep burial dissolution of Lower Palaeozoic carbonates and the role of compacted released water from Palaeogene strata in the Zhuanghai area, Jiyang Depression, Bohai Bay Basin, NE China

The Zhuanghai area is located in the northern part of the Jiyang Depression, Bohai Bay Basin, which is a superimposed basin. In this basin, there are very thick Cenozoic sediments that unconformably overlie Lower Palaeozoic rocks, which form buried hills (topographic highs). Through core description and microscopic observations, the diagenetic characters, pore types, reservoir quality and the dissolution characteristics of the Lower Palaeozoic strata have been investigated in the study area. Organic and inorganic gas–liquid bi‐phase inclusions with bright orange‐yellow fluorescence are evidence of the dissolution of the compacted released water enriched in organic matter released from Palaeogene strata into the Lower Palaeozoic strata. The relatively high homogenization temperatures of fluid inclusions can help explain the temperature condition of secondary low‐temperature hydrothermal minerals and the effects of dissolution of the compacted released water on the Lower Palaeozoic carbonates and hydrocarbons within the buried hills. The δ13C values of crystalline calcite in the high‐angle fractures of Lower Palaeozoic rocks range from −0.08‰ to −3.5‰, well above those of primary carbonates of the Lower Palaeozoic. The very high values of δ13C of crystalline calcite suggest the effects of dissolution of the compacted released water on the Lower Palaeozoic carbonates. The paper analyses the effects of dissolution of the compacted released water and the character of the reservoir space of the Zhuanggu‐10 well in the Zhuangxi Buried Hill and suggests that the Ordovician Majiagou Formation underwent significant dissolution because of the introduction of compacted released water and formed various secondary pores acting as a primary reservoir. The compacted released water that corroded the Lower Palaeozoic carbonates comes from the Palaeogene sediments in the adjacent Zhuangxi Sag. It migrated along the boundary fault of the sag to the Zhuangxi Buried Hill, modified the Lower Palaeozoic carbonates and caused the formation of deeply buried dissolution pores. The dissolution mode of the down‐dip faulted block seepage as recognized in the Chengbei 305 to 307 wells is proposed to explain the pore distribution patterns of the buried hills in the research area. Copyright © 2015 John Wiley & Sons, Ltd.

Geochronological and geochemical constraints on the origin of Yaguila Cretaceous and Palaeogene ore‐bearing quartz porphyries, Central Lhasa Terrane, Tibet

The quartz porphyries at Yaguila, Tibet, host important polymetallic ore deposits, but their age, source and evolution have received insufficient attention. In this study, LA‐ICP‐MS U–Pb zircon ages revealed that these quartz porphyries were formed in two stages, i.e. pre‐collisional Early Cretaceous period (ca. 128 Ma) and late subduction to initial collision Palaeogene period (ca. 65 Ma). Geochemical evidence including whole‐rock major element, trace element and rare earth element, zircon Hf isotopes together with sulphide Pb isotopes suggests the Early Cretaceous quartz porphyries could have formed by means of partial melting of crustal materials from Lhasa Terrane basement in a thickened crustal tectonic setting, whereas the Palaeogene quartz porphyries by enhanced input of mantle components probably were associated with slab rollback and breakoff at that time. Granitoid magmas produced in this way ascended to upper crustal level and interacted with marbles and breccias to form Pb–Zn–Ag ore bodies and Mo ore bodies, respectively. Age and geochemical constraints on the Early Cretaceous and Palaeogene polymetallic ore bodies not only have favoured a skarn and hydrothermal vein origin over a submarine exhalative sedimentary origin but also have consummated ore deposits metallogenic series and built up a reconnaissance model along the Lhasa Terrane. Copyright © 2015 John Wiley & Sons, Ltd.