РАННЕПЕРМСКИЙ (300–270 МЛН ЛЕТ) МАГМАТИЗМ ВОСТОЧНОГО КАЗАХСТАНА КАК РЕЗУЛЬТАТ СОЧЕТАНИЯ ПЛЕЙТ- И ПЛЮМ-ТЕКТОНИЧЕСКИХ ФАКТОРОВ

The history of the Central Asian Orogenic Belt (CAOB) was marked by several major events of magmatism which produced large volumes of volcanic and intrusive (mafic-ultramafic and granitic) rocks within a relatively short time span (30–40 Ma) over a vast area. The magmatic activity postdated the orogenic stages of accretionary-collisional belts in Central Asia and likely resulted from the impact of mantle plumes that formed Large Igneous Provinces (LIPs). The formation of the Tarim–South Mongolia LIP at 300–270 Ma is the best known among the major Permian events of basaltic and granitic magmatism. Early Permian igneous rocks (volcanic, subvolcanic and intrusive suites that vary from ultramafic to felsic compositions) of the same age range (300 to 270 Ma) have been recently found also in Eastern Kazakhstan, within the late Paleozoic Altai collisional system. The compositions and ages of the rocks suggest that the Eastern Kazakhstan magmatism was the northward expansion of the Tarim LIP. The spread of the Tarim LIP was apparently facilitated by lithospheric extension after the Siberia-Kazakhstan collision. The extension led to rheological weakening of the lithosphere whereby deep mantle melts could penetrate to shallower depths. The early Permian history of Eastern Kazakhstan was controlled by the interplay of plate tectonic and plume processes: plate-tectonic accretion and collision formed the structural framework, and the Tarim mantle plume was a heat source maintaining voluminous magma generation.В истории развития крупнейшего Центрально-Азиатского складчатого пояса (ЦАСП) выявлены несколько периодов крупномасштабной эндогенной активности, характеризующихся проявлениями значительных объемов вулканических и интрузивных (как базитовых, так и гранитоидных) пород на обширных территориях в сравнительно короткие временные интервалы (30–40 млн лет). Эти вспышки магматической активности обычно происходят после завершения аккреционно-коллизионных процессов в складчатых системах и рассматриваются как результат воздействия мантийных плюмов на литосферу – крупные изверженные провинции. Одним из ярких примеров является Тарим-Южномонгольская крупная изверженная провинция (300–270 млн лет назад), характеризующаяся широким развитием базитового и гранитоидного магматизма в западной части ЦАСП. Исследования последних лет показали, что в Восточном Казахстане, в пределах Алтайской коллизионной системы герцинид, широко распространены как базитовые, так и гранитоидные комплексы раннепермского возраста (300–270 млн лет). В приведенном кратком обзоре показано, что особенности состава и условия формирования этих магматических ассоциаций позволяют рассматривать их как результат северо-западного распространения влияния Таримской крупной изверженной провинции. Распространение этого термического возмущения в литосфере,по-видимому, стало возможным благодаря пост-орогеническому растяжению после коллизии Сибирского и Казахстанского континентов. Реологическое ослабление литосферы позволило глубинным расплавам проникать в литосферную мантию, образовав крупные очаги базитовых магм. Таким образом, современный геологический облик и металлогеническая специфика территории Восточного Казахстана является результатом плейт-тектонических процессов посторогенического растяжения на фоне повышенного термического градиента в мантии, вызванного активностью Таримского мантийного плюма

РОЛЬ МАГМАТИЧЕСКИХ ИСТОЧНИКОВ ТЕПЛА ПРИ ФОРМИРОВАНИИ РЕГИОНАЛЬНОГО И КОНТАКТОВЫХ МЕТАМОРФИЧЕСКИХ АРЕАЛОВ ЗАПАДНОГО САНГИЛЕНА (ТУВА)

The tectonomagmatic evolution of the Sangilen massif has been described in detail in numerous publications, but little attention was given to heat sources related to the HT/LP metamorphism. Modeling of the magma transport to the upper‐crust levels in West Sangilen shows that the NT/LP metamorphism is related to gabbromonodiorite intrusions. This article is focused on the thermo‐mechanical modeling of melting and lifting of melts in the crust, taking into account the density interfaces. The model of the Erzin granitoid massif shows that in case of fractional melting, the magma ascent mechanism is fundamentally different, as opposed to diapir upwelling – percolation take place along a magmatic channel or a system of channels. An estimated rate of diapiric rise in the crust amounts to 0.8 cm/yr, which is more than an order of magnitude lower than the rate of melt migration in case of fractional melting (25 cm/yr). In our models, a metamorphic thermal ‘anticline’ develops in stages that differ, probably, due to the modes of crust melting: batch melting occurs at the first stage, and fractional melting takes place at the second stage. It is probable that the change of melting modes from melting conditions in a ‘closed’ system to fractional melting conditions in ‘open’ systems is determined by tectonic factors. For the Sangilen massif, we have estimated the degrees of melting in the granulite, granite, and sedimentary‐metamorphic layers of the crust (6, 15, and 5 vol. %, respectively).Тектономагматическая эволюция Сангиленского массива детально охарактеризована в многочисленных публикациях, в то время как источникам тепла при метаморфизме НТ/LР‐типа уделялось мало внимания. Моделирование процессов транспорта магм на верхнекоровые уровни является актуальным, так как для Западного Сангилена устанавливается связь метаморфизма НТ/LР‐типа с габбро‐монцодиоритовыми интрузиями. Статья посвящена результатам термомеханического моделирования порционного режима плавления и подъема расплавов в коре с наличием плотностных границ. Объект моделирования – Эрзинский гранитоидный массив. Показано, что в отличие от диапирового всплывания механизм подъема магмы при фракционном плавлении оказывается принципиально иным – в форме просачивания по магматическому каналу (системе кана‐ лов). Оценено, что скорости подъема диапиров в земной коре (0.8 см/год) более чем на порядок ниже скорости миграции расплава при фракционном плавлении, которая составляет 25 см/год. Показано, что этапы развития метаморфической термальной «антиклинали» могли быть обусловлены разным режимом плавления материала коры: на первом этапе –порционного типа, на втором –фракционного. Смена режимов плавления от условий плавления в «закрытой» системе к условиям фракционного плавления в «открытых» системах, вероятно, определялась тектоническими факторами. Сделаны оценки степени плавления в гранулитовом (6 об. %), гранитном (15 об. %) и осадочно‐метаморфическом (5 об. %) слое коры Сангиленского массива

A REVIEW OF EARLY PERMIAN (300–270 MA) MAGMATISM IN EASTERN KAZAKHSTAN AND IMPLICATIONS FOR PLATE TECTONICS AND PLUME INTERPLAY

The history of the Central Asian Orogenic Belt (CAOB) was marked by several major events of magmatism which produced large volumes of volcanic and intrusive (mafic-ultramafic and granitic) rocks within a relatively short time span (30–40 Ma) over a vast area. The magmatic activity postdated the orogenic stages of accretionary-collisional belts in Central Asia and likely resulted from the impact of mantle plumes that formed Large Igneous Provinces (LIPs). The formation of the Tarim–South Mongolia LIP at 300–270 Ma is the best known among the major Permian events of basaltic and granitic magmatism. Early Permian igneous rocks (volcanic, subvolcanic and intrusive suites that vary from ultramafic to felsic compositions) of the same age range (300 to 270 Ma) have been recently found also in Eastern Kazakhstan, within the late Paleozoic Altai collisional system. The compositions and ages of the rocks suggest that the Eastern Kazakhstan magmatism was the northward expansion of the Tarim LIP. The spread of the Tarim LIP was apparently facilitated by lithospheric extension after the Siberia-Kazakhstan collision. The extension led to rheological weakening of the lithosphere whereby deep mantle melts could penetrate to shallower depths. The early Permian history of Eastern Kazakhstan was controlled by the interplay of plate tectonic and plume processes: plate-tectonic accretion and collision formed the structural framework, and the Tarim mantle plume was a heat source maintaining voluminous magma generation

THE ROLE OF MAGMATIC HEAT SOURCES IN THE FORMATION OF REGIONAL AND CONTACT METAMORPHIC AREAS IN WEST SANGILEN (TUVA, RUSSIA)

The tectonomagmatic evolution of the Sangilen massif has been described in detail in numerous publications, but little attention was given to heat sources related to the HT/LP metamorphism. Modeling of the magma transport to the upper‐crust levels in West Sangilen shows that the NT/LP metamorphism is related to gabbromonodiorite intrusions. This article is focused on the thermo‐mechanical modeling of melting and lifting of melts in the crust, taking into account the density interfaces. The model of the Erzin granitoid massif shows that in case of fractional melting, the magma ascent mechanism is fundamentally different, as opposed to diapir upwelling – percolation take place along a magmatic channel or a system of channels. An estimated rate of diapiric rise in the crust amounts to 0.8 cm/yr, which is more than an order of magnitude lower than the rate of melt migration in case of fractional melting (25 cm/yr). In our models, a metamorphic thermal ‘anticline’ develops in stages that differ, probably, due to the modes of crust melting: batch melting occurs at the first stage, and fractional melting takes place at the second stage. It is probable that the change of melting modes from melting conditions in a ‘closed’ system to fractional melting conditions in ‘open’ systems is determined by tectonic factors. For the Sangilen massif, we have estimated the degrees of melting in the granulite, granite, and sedimentary‐metamorphic layers of the crust (6, 15, and 5 vol. %, respectively)

In-Situ Crystallization and Continuous Modification of Chromian Spinel in the "Sulfide-Poor Platinum-Group Metal Ores" of the Norilsk-1 Intrusion (Northern Siberia, Russia)

Layers rich in chromian spinel (Cr-spinel) occur in numerous differentiated and layered intrusions. These layers are often characterized by elevated and even economic concentrations of platinum-group-elements (PGEs), but only scarce sulfide mineralization. One particular type of such lithology occurs in the roof parts of the Norilsk-type differentiated intrusions (Russia) and is referred to as the "sulfide-poor PGE ores". We investigated rocks containing variable enrichments in Cr-spinel, sulfides, and platinum-group minerals (PGMs) from two sections of the upper zone of the Norilsk-1 intrusion, with a focus on Cr-spinel. The rocks are dominated by two lithological types: (1) leucogabbro/troctolitic and (2) olivine-gabbro. Fine-grained (5-100 mu m) disperse disseminations with varying modal abundances of Cr-spinel are characteristic for the rocks studied. Those abundances range from scarce mineralization through to very dense (up to 30 vol. % Cr-spinel) cloud-like accumulations. However, compact-grained accumulations and cumulate-like textures, which are typical for chromitites of layered intrusions, are not characteristic for the studied rocks. Instead, the disseminations exhibit chain- and trail-like alignments of Cr-spinel grains, which cross the boundaries between enclosing silicates, and sub-circular arrangements. The study revealed millimeter-scaled patchy distribution of Cr-spinel compositions within a given dissemination with Cr-spinel chemistry being strongly correlated with a kind of the enclosing silicate. (1) In unaltered rocks, plagioclase hosts more magnesian Cr-spinel (Mg# 30-60), while Cr-spinel in mafic minerals is less magnesian (Mg# 18-35). (2) In altered rocks, more magnesian Cr-spinel is hosted by less altered silicates, while strongly altered silicates mainly host less magnesian Cr-spinel. Systematics of trivalent cations exhibits divergent trends, even on a scale of a thin section, and depends on a kind of hosting lithology. Leucogabbro/troctolite lithologies contain Cr-spinel with anomalously low Fe(3+)and extremely high Ti contents, whereas Cr-spinel from olivine-gabbro lithologies have moderate Fe(3+)and moderately-high Ti contents. It is envisaged that crystallization of Cr-spinel and their host rocks occurred from viscous mingled magmas, which had different compositions and redox state. Subsequent processes involved (1) high-temperature re-equilibration of Cr-spinel with enclosing silicates and (2) post-magmatic alteration and partial recrystallization of Cr-spinel. During these processes, Cr-spinel was losing Mg and Al and gaining Fe and Ti. These chemical trends are generally coincident with those established for other intrusions worldwide, but the upper zone of the Norilsk-1 intrusion seems to possess an exceptional variety of Cr-spinel compositions, not recorded elsewhere.Peer reviewe