Paleozoic Granites Research Articles

Geochronological, elemental and Sr-Nd-Hf-O isotopic constraints on the petrogenesis of the Triassic post-collisional granitic rocks in NW Thailand and its Paleotethyan implications

New U-Pb geochronological, petrologic, elemental and Sr-Nd-Hf-O isotopic data for the granites from the Inthanon and Sukhothai zones in NW Thailand in conjunction with correlations with SW China are presented to constrain the age and position of the Paleotethys Ocean in this region and the associated assembly of Southeast Asia. The geochronological data show that the granitic rocks in the Inthanon and Sukhothai zones, herein named Group 1 and Group 2 granites, respectively, yield similar crystallization ages of 230–200Ma. Group 1 samples are characterized by monzogranite and granite with I- and S-type geochemical affinity and Group 2 samples by I-type monzogranite and granodiorite. They have generally similar chondrite-normalized REE and PM-normalized multi-element patterns but distinct Sr-Nd-Hf-O isotopic compositions. Group 1 samples have slightly higher initial 87Sr/86Sr ratios (0.7111–0.7293) but lower εNd(t) values (−11.1 to −14.1) than those of Group 2 samples (87Sr/86Sr(i)=0.7073–0.7278 and εNd(t)=−8.3 to −11.0). Group 1 samples show the lower εHf(t) values (−5.4 to −18.2), older TDM (1.62–2.40Ga) and higher δ18O values (+7.95 to +9.94) than those of Group 2 samples (εHf(t) of −11.1 to +4.80, TDM of 0.96–1.95Ga and δ18O of +4.95 to +7.98) for the Triassic crystallization zircons. These geochemical signatures are similar to the Kwangsian and Indosinian granites in the South China and Indochina blocks but distinct from those of the Gangdese I-type granite and Sibumasu Paleozoic granite. Our data suggest that Group 1 samples mainly originated from the early Paleozoic supracrustal rocks containing metapelite and metavolcanic components, which had previously experienced the surface weathering. Group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component. Synthesis of our data with available regional observations indicates that the Inthanon zone represents the main suture zone of the eastern Paleotethyan Ocean in NW Thailand and links with the Changning-Menglian suture zone in SW Yunnan (SW China). In NW Thailand, a switch from the eastward subduction of the Paleotethyan oceanic plate to the collision of the Sibumasu with Indochina blocks occurred at ~237Ma, and syn- and post-collisional time being at ~237–230Ma and ~200–230Ma, respectively. The late Triassic granites in the Inthanon and Sukhothai zones are representative of the post-collisional magmatic products.

Miocene structural evolution and exhumation of the Ximeng dome in Yunnan, southeastern Tibet: Implications for intraplate deformation during extrusion of the Sundaland block

The Ximeng complex is located in the eastern Shai Thai block, a part of the extruding Sundaland block in southeastern Tibet. The complex is mainly composed of Neoproterozoic medium-grade metamorphic rocks and Early Paleozoic granite in the core surrounded by Paleozoic low-grade metamorphic rocks as a cover. The two structural units are separated by a tectonic contact (TDC). Detailed structural analysis reveals that the structural framework of the Ximeng complex is a dome structure and the complex experienced a primary and progressive shearing during doming. The rheological stratification led to the differential behavior of rocks from different stratigraphic and crustal levels during shearing. Zircon U-Pb ages of the Laojiezi granites in the core range from 465.50±2.70 to 430.37±0.93Ma. The cooling path from 40Ar-39Ar and apatite fission track data indicates that the complex experienced two stages of Cenozoic cooling, i.e., an early rapid cooling and a subsequent slow cooling. The early rapid cooling and exhumation of the complex are attributed to doming and accompanied by middle to upper crustal flow from ca. 23 to 20Ma.Shearing, cooling and exhumation of the middle to lower crustal rocks are widely distributed along the boundary fault zones (i.e., the Ailao Shan Red River shear zone and the Sagaing shear zone) of the extruding Sundaland block and within the triangular area between them. The doming, ductile shearing and exhumation of the Ximeng complex highlight the importance of intraplate deformation coeval with Indian-Eurasian collision. Localized crustal flow within the Sundaland block is possibly an important process during the extrusion of the Sundaland block.

Alpine tectonics of granites in basement of Ysyk-Köl Basin, northern Tien Shan

The Ysyk-Kol Basin filled with Lower Jurassic–Quaternary sedimentary rocks is the largest intermontane negative structural unit of the northern Tien Shan. The basement of this basin is composed of Precambrian–Paleozoic rocks, largely of Ordovician and Silurian granitoids exposed in mountain ranges of the basin framework and as separate anticlinal domes situated in areas occupied by the Mesozoic–Cenozoic sedimentary cover. The postmagmatic tectonic internalstructure of the Chonkurchak (Chunkurchak), Kyzyl-Choku, Kyzyl-Bulak, and Prishib massifs emplaced in the basement, as well as their relationships to the sedimentary cover, are described in the paper. The study was carried out using the morphostructural method, detailed geological mapping, structural kinematic analysis, and petrographic examination of rocks. The internalstructure of Paleozoic granites in the basement and indications of their 3D tectonic flow are characterized. It is shown that granites underwent 3D deformation after their emplacement in the consolidated crust, and this process had a substantial influence on tectonic processes at the plate and orogenic stages of regional evolution.

Ore sources of the base-metal veins at the Dzhimidon deposit, North Ossetia, Russia: Evidence from REE distribution in ores and host rocks

REE distribution patterns of the ores and host rocks of the Dzhimidon vein lead-zinc deposit (North Caucasus, Ossetia, Sadon mining district, Russia) have been analyzed to estimate ore sources of hydrothermal deposits. It is established that both types of prevailing rocks of the Sadon Area are involved in the formation of base-metal ores during activation of hydrothermal metasomatic processes in the Jurassic time, the said rocks are Paleozoic granites (the main ore-hosting rocks at the majority of deposits) and Pre- Cambrian schists (specific for only ore-hosting rocks of Dzhimidon deposit).

Age and petrogenesis of late Paleozoic granites from the northernmost Alxa region, northwest China, and implications for the tectonic evolution of the region

The Wudenghan, Huhetaoergai and Zhuxiaobuguhe plutons, northern Alxa region, in the southern Central Asia Orogenic Belt are dated by U–Pb zircon to 383 ± 3, 356 ± 3 and 286 ± 2 Ma, respectively. The late Devonian Wudenghan monzogranite, a highly fractionated I-type granite with ε Nd(t) (−0.2 to −0.1) and very low (87Sr/86Sr) t (0.704719–0.706113), is from mantle-derived magmas and shows volcanic arc characteristics. The early Carboniferous Huhetaoergai granodiorite with medium-K calc-alkaline peraluminous characteristics represents a volcanic arc granite generated from partial melting of lower continental crust combined with mantle-derived input. The early Permian Zhuxiaobuguhe pluton, an unfractionated calc-alkaline granodiorite with moderately low ε Nd(t) (−2.0 to −1.1) and low (87Sr/86Sr) t (0.708370–0.708462), was likely derived from partial melting of the mafic lower crust of a paleo-volcanic arc and represents a post-collisional granite. Our revised tectonic evolution of the region includes (1) northward subduction of the oceanic crust represented by the Engger Us Ophiolitic Belt and formation of the late Devonian Wudenghan monzogranite, (2) northward subduction of the ocean between the Huhetaoergai and Zhusileng tectonic zones and the formation of the Huhetaoergai volcanic arc granite during the early Carboniferous and (3) the emplacement of the Zhuxiaobuguhe pluton in the early Permian during post-collisional extension.

Ore geology, fluid inclusion and 40Ar/39Ar geochronology constraints on the genesis of the Yingchengzi gold deposit, southern Heilongjiang Province, NE China

The Yingchengzi gold deposit, located at the eastern margin of the Zhangguangcai Range, southern Heilongjiang Province, NE China, is the only high-grade gold deposit in the region. The deposit is hosted in mylonitized Paleozoic granite and metavolcanic rocks and is structurally dominated by brittle–ductile shear zones and faults. The orebodies consist of gold-bearing quartz veins and gold-bearing altered mylonites. The hydrothermal ore-forming process can be divided into early, middle and late stages, characterized by quartz–pyrite veins, quartz–gold–polymetallic sulfide veins, and quartz–carbonate veins or veinlets, respectively. Ore minerals and gold mainly occurred in the middle stage.Four types of fluid inclusions were identified in the vein minerals based on petrography and laser Raman spectroscopy: NaCl–H2O (W-type), CO2–H2O (C-type), pure CO2 (PC-type) and less common calcite-bearing (S-type) inclusions. The early stage minerals contain C-type and rare PC-type fluid inclusions which have homogenization temperatures of 262–400°°C, and low salinities of 2.20–8.51wt.% NaCl equiv. Only the middle stage minerals contain all four types of fluid inclusions with homogenization temperatures of 173–316°°C, and salinities of 2.20–12.42wt.% NaCl equiv. The late stage quartz and calcite crystals contain only W-type fluid inclusions, which have homogenization temperatures of 104–207°°C and salinities of 1.05–6.58wt.% NaCl equiv. Fluid boiling and the mixing caused the rapid precipitation of sulfides and gold. These indicate that the ore-forming fluid system evolved from CO2-rich, metamorphogenic to CO2-poor, due to the boiling and input of meteoric water. The trapping pressures estimated from C-type fluid inclusions are 98–245MPa in the main ore-forming stage, giving a reasonable estimate that an alternating lithostatic–hydrostatic fluid system controlled a fault-valve activity at a depth of 9–10km.40Ar/39Ar dating of hydrothermal sericite from gold-bearing altered rock shows that gold mineralization occurred at ~248Ma. We assume that the Yingchengzi gold deposit was formed in a Late Permian–Early Triassic continental collision regime following closure of the Paleo-Asian Ocean. The regional geology, ore geology, fluid inclusions, mineralization age and tectonic setting infer that the Yingchengzi gold deposit is a mesozonal orogenic gold system, making it the first orogenic gold deposit discovered in the Zhangguangcai Range, NE China.

Structure of the Pliocene Camp dels Ninots maar-diatreme (Catalan Volcanic Zone, NE Spain)

Maar volcanoes expose shallower or deeper levels of their internal structure as a function of the degree of erosion. In El Camp dels Ninots maar-diatreme (Catalan Volcanic Zone, Spain), the tephra ring has been largely eroded, and the remaining volcanic deposits infilling the diatreme are hidden under a lacustrine sedimentary infill of the crater. The volcano shows hardly any exposure, so its study needs the application of direct (e.g., boreholes) and indirect (shallow geophysics) subsurface exploration techniques. Additionally, this maar-diatreme was built astride two different substrates (i.e., mixed setting) as a result of its location in a normal fault separating Neogene sediments from Paleozoic granites. In order to characterize the internal structure and post-eruption stratigraphy of the maar-diatreme, we did geological studies (mapping, continuous core logging, and description of the tephra ring outcrops) and near-surface geophysics, including nine transects of electric resistivity tomography and a gravity survey. Results show that the deeper part of the diatreme is excavated into granites and is relatively steep and symmetrical. The uppermost diatreme is asymmetrical because of mechanical contrast between granites and Pliocene sands. The maar crater contained a lake permanently isolated from the surrounding relief and was deep enough to host anoxic bottom waters while its margins had shallower waters. These lake conditions preserved the remarkable Pliocene fossil record found in the lacustrine sediments.

Finding of 115 Ma adakitic tonalite east of the Futaba Fault, Abukuma Highland, and its regional geological implications

©The Geological Society of Japan 2015 167 Abstract: We conducted U–Pb dating on zircon crystals separated from two drill core samples to ascertain whether a tonalite body discovered by drilling in Matsukawaura east of the Futaba Fault near the city of Soma, belongs to a 300-Ma Paleozoic granite, or to Cretaceous granitoids with age common to the Abukuma Highlands or the Kitakami Mountains. The zircons yield U–Pb ages of 115–114Ma, which are slightly older than the age of Abukuma granitoids exposed to the area between the Hatagawa and Futaba faults (108– 106Ma). The Matsukawaura tonalite, having a typical adakitic signature, exhibits similarity to Cretaceous Takase Granite in Wariyama Sheared Granodiorite occurring along the east side of the Futaba Fault. Report

Geochronology and geochemical constraints on petrogenesis of Early Paleozoic granites from the Laojunshan district in Yunnan Province of South China

The Early Paleozoic Orogen in South China has been a topic of hot debate; in particular with regard to the relative contributions of crustal and mantle sources to the generation of granites. To address this issue, we report geochronological and geochemical data for three related suites of granites (Tuantian, Suite 1, Nanlao, Suite 2, and Laochengpo, Suite 3) from the Laojunshan district in the southeast Yunnan Province, western South China Block (SCB). LA-ICP-MS U-Pb dating of zircons indicates magmatic crystallization ages of ~436Ma, ~430Ma and ~427 Ma for Suites 1, 2, and 3 granites, respectively. The εHf(t) values of magmatic zircons in all three suites are heterogeneous, ranging between +3 and −14. All three suites also contain inherited zircon cores with Proterozoic U-Pb and Hf model ages. Bulk compositions form well-defined mixing arrays, with Suites 1 and 2 being strongly peraluminous (A/CNK >1.1), whereas Suite 3 is weakly peraluminous (A/CNK=1.0–1.1). Suites 1 and 2 have lower TFe2O3, Al2O3, MnO, MgO, CaO, TiO2, Na2O, Mg# and Nb/Ta but higher K2O, Rb/Sr, Rb/Ba and εNd(t) than those of Suite 3. These results suggest that Suite 1 and Suite 2 were formed by partial melting of Proterozoic metasedimentary rocks with little or no input of mantle-derived materials, but Suite 3 was derived from a mixture of a crustal melt with mantle derived mafic magma. The apparent paradox that the mantle-derived component has lower εNd(t) values than the crustal components is consistent with the very low εNd(t) values found in mantle-derived basalts in the SCB. The Wuyi-Yunkai orogenic collapse may have caused lower crustal melting of the Cathaysia Block. The early Paleozoic granitoid magmatism with mafic magma input indicates an extensional environment in the western SCB, in response to post-collisional orogenic collapse. Asthenospheric upwelling and basaltic underplating probably contributed heat and melts triggering felsic magmatism in the western SCB.

Geochronological framework and tectonic setting of the granitic magmatism in the Chaihe–Moguqi region, central Great Xing’an Range, China

We compiled and analyzed 56 U–Pb zircon ages, including 38 ages obtained during the present study and 18 from other sources, for granitic rocks of the Chaihe–Moguqi region, central Great Xing’an Range, China. Magmatism in this area can be divided into eight stages: Early Devonian (399±3Ma), Late Devonian (365–358Ma), Late Carboniferous (322–299Ma), Early Permian (295–282Ma), Late Triassic (231–227Ma), Early–Middle Jurassic (179–172Ma), Late Jurassic (152–149Ma) and Early Cretaceous (137–120Ma). Granites were mainly emplaced during the latter four (Mesozoic) stages of magmatism. Paleozoic granites formed during several stages, and were associated with oceanic subduction and the amalgamation of crustal blocks in the eastern fragment of the Central Asian Orogenic Belt. The Late Triassic is an important period with respect to the change from the Paleo-Asian Ocean tectonic regime to the circum-Pacific tectonic regime. The formation of Late Triassic granites may have been related to post-orogenic lithospheric extension after closure of the Paleo-Asian Ocean. Early–Middle Jurassic granites resulted from either subduction of the Paleo-Pacific Plate or subduction during the amalgamation of the Jiamusi Massif and the Songliao terrane, whereas the intrusion of Late Jurassic–Early Cretaceous granites was associated with both the subduction of the Paleo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean.

Evidence for a far-traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467 + 7/ − 10 Ma and ~570 Ma, with metamorphic zircon growth at 23–29 Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475 + 7/ − 3 and ~660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile “thrust sheets,” present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.

The Gondwana connection of South China: Evidence from monazite and zircon geochronology in the Cathaysia Block

The tectonic evolution of the Cathaysia Block in South China has been debated, with limited information to constrain the early Paleozoic geodynamics and paleogeography in the context of the global Gondwana supercontinent. In this study, we report the petrology of mafic magmatic suites and metamorphic units, together with zircon and monazite U–Pb ages from several key areas in Cathaysia. Combined with a comprehensive synthesis of the data reported in previous studies, we argue that the Cathaysia was an early Paleozoic collisional orogen, and speculate the possible linkage of South China with the Gondwana supercontinent. The monazite and zircon U–Pb data presented in this study including the metamorphic ages of the rocks in the Gaozhou complex in Yunkai massif, the Wanquan Group in central Wuyi massif, the garnet–sillimanite gneiss from the Chencai Group, and the garnet amphibolite from the Longyou Group in the northeastern section of Wuyi massif show ages ranging from 447Ma to 430Ma. Gabbroic intrusions in the Longyou Group and pyroxenites in the Chencai Group in northeastern Wuyi massif were emplaced at ca. 425Ma and 470Ma, respectively. Furthermore, the ca. 470Ma pyroxenite was overprinted by hydrothermal alteration at ca. 400Ma. Whole-rock geochemistry and mineral compositions show that the Chencai pyroxenite possesses features similar to that of the Alaskan-type mafic intrusions, whereas the 420Ma gabbroic intrusions in the Long Group exhibit post-collision signature, which are consistent with the broadly coeval voluminous post-collision granites in Cathaysia. Comprehensive synthesis of zircon and monazite U–Pb ages of the metamorphic rocks and granites in Cathaysia leads us to suggest that the transition from collision to post-collision occurred at ca. 430Ma.The early Paleozoic structures, metamorphism and magmatic activity suggest that the NE-extending Cathaysia was a typical collisional orogenic belt rather than an intraplate type. The predominance of late Neoproterozoic–Cambrian detrital zircons in the early Paleozoic granites and pre-Devonian metamorphic rocks in Cathaysia indicates that at least some clastics of the lower Paleozoic rocks in Cathaysia were derived from the Gondwana continents, suggesting that South China was at the fringe of the Gondwana supercontinent. The Nanhai terrane (including the Hainan massif) could represent the missing link between South China and Gondwana, with the Zhenghe–Dapu fault marking the suture zone between South China and the Nanhai terrane.

New age data refine extent and duration of Paleozoic and Neoproterozoic plutonism at Ganderia–Avalonia boundary, Newfoundland

Devonian and younger plutonic suites of the Appalachian orogen in Newfoundland form a curvilinear belt straddling the boundary between Ganderia and Avalonia, and exhibit a general pattern of younging to the southeast. However, many individual plutonic suites within this belt remain undated or have ages that are poorly constrained. This study reports U–Pb sensitive high-resolution ion microprobe (SHRIMP) data from zircon and 40Ar–39Ar data from hornblende, biotite, and muscovite, which collectively improve definition of post-tectonic plutonic events in southeastern Newfoundland, but at the same time raise new questions for further study. U–Pb zircon SHRIMP determinations provide valuable new geochronological information for the Deadman’s Bay Granite (390 ± 4 Ma), the Gander Lake Granite (378 ± 4 Ma), the Terra Nova Granite (384 ± 4 Ma), the Old Woman stock (376 ± 4 Ma), and the easternmost unit (Tolt phase) of the composite Ackley Granite (372 ± 4 Ma). The Pass Island Granite, long considered part of this age group, gave a Silurian age of 423 ± 4 Ma and provides the first confirmation of Silurian granitoid magmatism in Avalonia of Newfoundland. The Berry Hills Granite on the Burin Peninsula, previously designated as Devonian, instead gave a Neoproterozoic age of 570 ± 6 Ma, and a similar age of 575 ± 6 Ma was obtained from granite that is intruded by the ca. 376 Ma Old Woman stock. These results indicate that regional geological context, deformation state, and (or) petrographic–geochemical data are not reliable indicators of emplacement age in this region. 40Ar–39Ar data from the Paleozoic granites provide mixed results in comparison to the U–Pb emplacement ages, and in some cases Ar systematics appear to have been disturbed by later hydrothermal alteration. In other cases, correspondence between the U–Pb and 40Ar–39Ar data sets is consistent with rapid postemplacement exhumation. Granophile-type mineralization (Mo, W, F, base metals) is present within both Ganderia and Avalonia examples, but is generally restricted to the youngest plutons, in which it is associated with high-level, near-roof, or apical settings. This age versus mineralization pattern is consistent with an interpretation in which mineralized environments within older suites have been preferentially eroded as a consequence of greater exhumation. The identification of a Silurian (ca. 423 Ma) granite in an area previously assigned to Avalonia may be important in constraining tectonic models for Ganderia–Avalonia interaction, which has generally been considered Silurian or younger. Given that many other plutonic suites in Avalonia are not well constrained by regional geology and remain undated, more work is needed to better define the extent of such magmatism and evaluate its implications for broader models of orogenic development.

Geochronology, elemental and Nd–Hf isotopic geochemistry of Devonian A-type granites in central Jiangxi, South China: Constraints on petrogenesis and post-collisional extension of the Wuyi–Yunkai orogeny

Abstract Details on processes of the early-middle Paleozoic Wuyi–Yunkai orogeny in South China remain poorly defined. Most Silurian–Devonian granites in South China are S-type or I-type granites, which are suggested to be petrogenetically related to the Wuyi–Yunkai orogeny. This paper firstly reported a systematic study on two Devonian A-type granites in the central Jiangxi Province. LA-ICP-MS zircon U–Pb dating results imply that the Huitong and Epo granites were emplaced at about 415 Ma. Both of the two granites have the petrographic and geochemical characteristics of A-type granites. Interstitial biotites occur along the boundary of euhedral plagioclase and quartz and they formed later than plagioclase and quartz. It implies that the primary magma could have been anhydrous. Biotites from the two granites are Fe-rich and have high Fe 2 + /(Fe 2 + + Mg 2 + ) ratios (0.60–0.74). The magmatic temperatures estimated from zircon saturation thermometer are 802–920 °C for the two granites, higher than common I-type and S-type granites. The two granites show high contents of total alkalis (Na 2 O + K 2 O = 6.96–9.39 wt.%), high field strength elements (e.g. Zr = 181–437 ppm, Y = 22.1–39.7 ppm, Nb = 18.6–30.3 ppm and Zr + Nb + Ce + Y = 324–555 ppm), rare earth elements (total REE = 155–312 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 2.50–3.44). These geochemical characteristics are similar to those of A-type granites. The Huitong and Epo granites have relatively low e Nd (t) values of − 10.4 to − 7.7, and low zircon e Hf (t) values (peak value of − 8.0). Whole-rock two stage Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.78 Ga to 2.00 Ga. According to these data, we suggest that the two granites might have derived from partial melting of pre-Cambrian sedimentary rocks which had been granulitized during an earlier thermal event. The two granites contain abundant contemporaneous mafic microgranular enclaves (MMEs). The MMEs display igneous textures and contain acicular apatites, suggesting quenching of mafic magmas that have co-mingled with the host granites. The MMEs might have derived from the enriched lithospheric mantle source. This study on the Devonian Huitong and Epo A-type granites, together with previous studies on Paleozoic granites in South China, indicated that the Wuyi–Yunkai orogeny in South China had changed from syn-collisional crustal thickening to post-collisional extension at least from 415 Ma.

Sources and geodynamic environments of formation of Vendian-Early Paleozoic magmatic complexes in the Daribi range, Western Mongolia

Rock complexes composing the Daribi Range were produced in Late Vendian, Early Cambrian, and Early Paleozoic suprasubduction systems. All of the studied mafic and ultramafic magmatic mantle rocks (the post-Vendian ophiolite complex, Early Cambrian pillow basalts, and Early Paleozoic picrobasalts of the sill-dike complex) have geochemical characteristics typical of early evolutionary episodes of island arcs: low LILE concentrations, horizontal REE patterns or patterns close to those of N-MORB, and HFSE minima. The magmas were derived from depleted mantle sources of variable isotopic composition with ɛNd(T) from +2.5 to +10. The Early Paleozoic rocks of the sill-dike complex were likely produced by a complicated interaction of melts derived from different sources. The rocks of group 1 resulted from the mixing of low-K picrite and tonalite melts. The picrite melts with ɛNd(T) from +6 to +8 were melted out of garnet lherzolite in the mantle wedge. The tonalite melts with ɛNd(T) = −3 seem to have been formed by the partial melting of mafic oceanic rocks of a subducted slab or the bottom of an island arc. The trondhjemite melts of group 2 with ɛNd(T) varying from 2.5 to 7.5 could be formed via the melting of subducted metapelites or amphibolites with low sulfide concentrations. Massifs of sodic Early Paleozoic granites also occur elsewhere in western Mongolia, Tuva, and the Altai territory. The generation of sodic silicic melts was likely a common process in supra-subduction systems in CAFB. The potassic granites (group 4) could be formed by the melting of subducted pelites or by the fractionation of mantle magmas. The genesis of the basaltic andesites (group 5) was likely related to Mesozoic-Cenozoic intraplate processes.

Paleomagnetism of the Late Paleozoic granites of the Angara-Vitim batholith and the host rocks of the Baikal-Patom folded area: Tectonic implications

The paper presents the results of paleomagnetic and geochronological studies of the Late Paleozoic granites of the Angara-Vitim batholith as well as Vendian-Early Cambrian sedimentary rocks and Late Devonian subvolcanic rocks of the Patom margin of the Siberian Platform. Primary and metachronous magnetization in the rocks of the study region was used to calculate an Early Permian (~290Ma) paleomagnetic pole, which is proposed as a reference pole for the Siberian Platform in paleomagnetic reconstructions, plotting of the apparent polar-wander path curve, and other magnetotectonic studies. The published and obtained paleomagnetic data and analysis of the geological data confirm the Late Paleozoic age of the final folding in the Baikal-Patom area. Possible causes of Late Paleozoic deformations and large-scale granite formation in the Baikal-Patom area and Transbaikalia in the Late Paleozoic are discussed.

Nanogold in ancient limestone and dolomite of the Oktyabr’skoe ore field (Priamurye)

692 The Oktyabr’skoe ore field (Priamurye) is located on the Precambrian protuberance of the basement. The Precambrian formations (Pt3 ?) occur as an extended band with a length of many kilometers, width of 2–4 km, and NW orientation and are retained as xenoliths in intrusions of Early Paleozoic granites. The upper horizons of the ancient series are composed of dolomite, limestone, and schist intruded by multi age Paleozoic and Mesozoic granitoids, ultrabasic rocks, and numerous dykes of Cretaceous age. Skarns and hornfels are registered in the exocontacts of intru sions. The zones of quartz metasomatic rocks are more abundant.

U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand — A new report of Early Paleozoic granite

Abstract In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th–U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas. This suggests that these three areas belong to the Sibumasu block, and the Sibumasu block records similar plutono-metamorphic history from Northern to Peninsular Thailand. Relative abundance of oceanic components in the Khao Chao gneiss, their Late Triassic magmatic ages, and the Early Jurassic metamorphic ages prefer the interpretation that the Khao Chao gneiss belongs to the Sukhothai Arc.

Factors Controlling Low Uranium and Thorium Concentrations in the Çayirhan Bituminous Shales (ÇBS) in the Beypazari (Ankara) Area, Turkey

AbstractIn this study uranium and thorium contents and depositional characteristics of Çayirhan bituminous shales (ÇBS), west of Ankara (central Anatolia), are investigated. Samples used were collected from boreholes opened by Park Holding Ltd. A total of 25 samples were taken from bituminous shale levels in boreholes drilled at 6 different locations in the study area. The Hirka formation which hosts bituminous shale deposits is a volcano sedimentary sequence and all lithofacies indicate that a lacustrine environment where the water level was continuously changed. In addition to hydrocarbon generation potential, bituminous shales also accumulate significant amount of radioactive elements such as uranium and thorium. The average uranium and thorium concentrations of ÇBS (1.83/2.62 ppm) are much lower than averages of UC, NASK and PAAS (uranium: 2.70/2.66/3.10 ppm; thorium: 8.50/12.30/14.60 ppm). Low uranium contents in comparison to those of similar lacustrine environments might be attributed to that waters of depositional environment of ÇBS contain low concentration of dissolved uranium and redox conditions were of oxic and dioxic character. Correlation data indicate that U and Th have a similar source and are associated predominantly with clays and phosphates and dominantly with organic material. Radioactive elements in the basin might be derived from Paleozoic granites and metamorphites (e.g. gneiss, schist) which comprise the basement and volcanism which was active in the region throughout the Miocene period. These elements are probably associated with uraniferous phosphate minerals (e.g. autunite, torbernite) which occur in granite, gneiss and schist. ÇBS with average TOC content of 10.96 % shows very good/perfect source rock potential. Positive correlations between Gamma‐Ray values and U, Th and K concentrations imply that radioactivity might be originated from these three elements.

Crustal Velocity Structure from the Northern Margin of the North China Craton to the Southern Margin of the Siberian Plate

AbstractThe survey of seismic refraction and wide‐angle reflection from Zhangjiakou to China‐Mongolia border is about 600 km long, which begins from the North China Plate, crosses Xing'an Mongolian orogenic belt and ends at the Siberian plate. The recordings of seismic wave field are composed of 8 shootings. Each shooting is generated from an explosion of 1.5 tons and received by 300 detectors along the survey. The qualities of received seismic wave field of P‐wave are good for most of the shootings. The data collected from the experiment of seismic refraction and wide‐angle reflection have been processed, analyzed and modeled. There are six seismic phases in the data: the arrivals of refracted and reflected seismic waves from the sediments and basements (Pg), the arrival of reflected seismic wave from the bottom of the upper crust (P2), the arrival of reflected seismic wave from the inner of middle crust (P3), the arrival of reflected seismic wave from the bottom of middle crust (P4), the arrival of reflected seismic wave from the inner of lower crust (P5) which exists only in the area between Xianghuang Qi and Sonid Youqi, and the arrival of reflected seismic wave from the Moho (Pm). The velocity distribution of crust is obtained using the seismic dynamics ray method (seis88). The results show that (1) there are local high velocity anomalies in the upper crust between the North China Plate and Inner Mongolia orogenic belt, as well as in the upper crust between the Inner Mongolia orogenic belt and Siberian plate, and there are also a lot of Paleozoic granite and ultramafic rock which are exposed at the surface of those places. (2) In the middle and lower crusts, the velocity of seismic wave in the northern margin of the North China plate is big, which is in the range of 6.3∼6.7 km/s, and in the southern margin of the Siberian Plate the velocity is relatively small and it is in the range of 6.1∼6.7 km/s. The velocity variation between the margins of the two areas is small. The reason for the velocity variations in different regions is that there exit the rugged undulation interfaces in middle and lower crusts, the splicing and substance exchanges between different land and sea blocks, which are caused by the crustal shortening and orogenic uplifting in the Inner Mongolia orogenic belt. (3) The Moho varies gently both in the south of F2 fault and in the north of F4 fault, and the depth of Moho is in the range of 40∼42 km. There exist double Moho boundaries between F2 and F4 faults. The Moho1 uplifts obviously, its depth is 33.5 km and the layer velocity is in the range of 6.6∼6.7 km/s. The Moho2 concaves downward obviously, the deepest boundary of it reaches 47 km and the layer velocity is in the range of 6.7∼6.9 km/s, which may be caused by the material exchange between the crust and mantle. Based on the characteristics of the Moho boundaries, the authors consider that the south area of double Moho boundaries is the northern margin of the North China plate, and the north area of them is the southern margin of the Siberian plate, the final position of suture between the North China Plate and Siberian plate locates in the area between Chifeng fault (F2) and Solon Obo‐Ar Horqin Qi fault (F4).