Geological data and about 300 precision geochronological and geochemical determinations are analyzed to identify the Albian–Cenomanian continental-margin orogenic belt and simultaneous...

Transform margins represent lithospheric plate boundaries with horizontal sliding of the oceanic plate, which in time and space replaced the subduction-related convergent margins. This happened due to the following: ridge-crest–trench intersection or ridge death along a continental margin (recent California and Baja California, Queen Charlotte–Northern Cordilleran, west of the Antarctic Peninsula, and probably Late Miocene–Pleistocene southernmost South America); change in the direction of oceanic plate movement (western Aleutian–Komandorsk and southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted from the spreading ridge collision (subduction) with a continental margin or slab tear formation after subduction cessation. Igneous magmatic series formed above the slab window or slab tear are similar in composition and show diversity of tholeiitic (sub-alkaline), alkaline, or even calc-alkaline and peraluminous rocks. The comprehensive geochemical dataset for igneous rocks (more than 2400 analyses) from the recent model geodynamic settings allowed us to build discriminant diagrams for the petrogenic oxides TiO₂ × 10–Fe₂O₃ ^Tot–MgO and trace elements Nb*5–Ba/La–Yb*10, which show distinctive rock features present on both convergent and Pacific-type transform margins. The author’s diagrams are capable of distinguishing volcanic and plutonic rocks formed above the subduction zones at an island arc and continental margin (related to convergent margins), from those formed in the strike-slip tectonic setting of transform margins along continents or island arcs.

This paper reports new geochronological data on metagranitoids (U–Pb SIMS) and ophiolites (Sm–Nd) from the Khanka massif. New and published data define the Early Neoproterozoic Matveevka–Nakhimovka terrane with 935- and 915-Ma early suprasubduction magmatism, 850–880-Ma and 757-Ma withinplate and Pacific-type transform margin magmatism, as well as the Late Neoproterozoic–Early Cambrian Dvoryan and Tafuin terranes with 543, 520, 517, and 513-Ma suprasubduction magmatism. These two terranes are separated by a suture (Voznesenka and Spassk terranes) formed by Ediacaran–Cambrian shelf deposits and a Cambrian accretionary wedge with ophiolites older than 514 Ma. The greater part of the Khanka massif formed in the late Cambrian, with the Kordonka island-arc terrane accreted at the end of the Silurian. The Sergeevka terrane of the Ordovician island arc joined it through the Early Cretaceous strike-slip movements. Heterogeneous structures of the main part of the Khanka massif can be traced to the north based on the analogous stages of magmatism and metamorphism, where the Jiamusi massif (including the East Bureya terrane) is an Early Neoproterozoic block and the eastern Songnen massif (including the West Bureya terrane) is a Late Neoproterozoic–Cambrian block. These blocks are separated by the Spassk–Wuxingzhen–Melgin suture formed by their collision in the Late Cambrian. The Bureya–Songnen–Jiamusi–Khanka superterrane formed as a part of the Gondwana supercontinent approximately 500 Ma ago through orogeny and accretion of the Rodinia supercontinent fragments.

Some geological, petrochemical, and geochemical characteristics of carbonaceous shales as a new unconventional natural source of gold and PGE are considered by the example of the Kimkan and Sutyr’ units of the Bureya massif (southern Far East, Russia). It is shown that shales of the units belong to the terrigenous-carbonaceous and siliceous-carbonaceous formations. They accumulated in deep-water trenches, and the active continental margin was probably their main provenance. The carbonaceous terrigenous-sedimentary units and precious-metal ores in them show specific petrochemical characteristics different for complexes with predominantly PGE and gold mineralization. According to these characteristics, carbonaceous complexes with high Fe contents, low total contents of alkalies, and high K/Na ratios are promising for PGE-rich ores. Gold ores are usually localized in black-shale strata with high total contents of alkalies and low K/Na. In this respect, the shales and Fe-ores of the Kimkan unit obviously contain high-PGE mineralization, while the rocks of the Sutyr’ unit can bear gold deposits. We assume that the PGE mineralization is genetically related to the formation and transformation of carbonaceous rocks. At the same time, most of gold in the carbonaceous shales is native and is not related to carbon; it is present in mineral assemblages resulted from superimposed sulfidization and silicification.

Testing of the geochemical compositions of the Late Cenozoic volcanic rocks of Kamchatka on new discriminant diagrams confirmed the existence of different geodynamic settings at that time. Late Miocene (~6 Ma)–Pliocene volcanic rocks of Eastern Kamchatka and the Central Kamchatka Depression, as well as the Late Pliocene (~3.5 Ma)–Holocene alkaline, calc–alkaline, and adakite volcanic rocks in the central part of the Sredinnyi Range are shown to be similar to the volcanic rocks of the Pacific-type transform margins. At the same time, the Miocene–Holocene volcanic rocks of Southern Kamchatka, the Miocene–Early Pliocene volcanic rocks of the Sredinnyi Range, and the Pleistocene–Holocene volcanic rocks of Eastern Kamchatka resemble the volcanic rocks of convergent margins. In central Kamchatka (from the coast to the Sredinnyi Range), igneous complexes typical of the transform margin were formed at the end of the Miocene–Pliocene, during the collision of the Kronotsky island-arc terrane and the movement of the Pacific plate. The geochemistry of the transform-margin volcanic rocks is caused by the upwelling of the subslab asthenosphere, both into the collision zone and the zone of the Sredinnyi Range volcanic arc, following the Commander–Kronotsky microplate slab segmentation and breakoff.