2016 Vol. 27, No. 3
The purpose of this paper is to determine the provenance and tectonic setting of the Phu Khat Formation and get a better understanding of the tectonic evolution of the Nakhon Thai region using the petrography and whole-rock geochemistry integrated with the U-Pb detrital zircon dating. The sandstone of the Late Cretaceous to Early Tertiary Phu Khat Formation is chiefly characterized by unsorted texture and highly unstable volcanic lithic fragments. The formation overlies unconformably on a high textural and mineral maturity of clastic sandstone of the Late Cretaceous Khao Ya Puk Formation. Geochemically, the tectonic setting discrimination (K2O/Na2O-SiO2, Al2O3/SiO2-Fe2O3+MgO, and Th-Sc-Zr/10) and the petrography indicate that the Phu Khat Formation was accumulated in a passive margin tectonic setting which is the same as the Khao Ya Puk Formation but with a different depositional environment. The plots of geochemical provenance discrimination (La/Th-Hf, Th/Sc-Zr/Sc, Eu anomaly Eu/Eu* 0.42 to 0.74) and the petrography reveal that the provenance of the Khao Ya Puk Formation is mainly recycled sedimentary rocks while the Phu Khat Formation consists primarily of recycled sedimentary rocks associated with minor felsic volcanic rocks from the old continental island arc of the uplifted either western or eastern continental terranes or both. However, the U-Pb detrital zircon dating indicates a unique provenance of the Phu Khat Formation from the terrane west of the Nakhon Thai region where the volcanic continental arc is active predominantly in the Middle to Late Triassic. The results indicate that while the Phu Khat Formation was accumulated in Nakhon Thai region, the western terrane was uplifted by reactivation of the preexisting structure probably since the Maastrichtian time to be the source area of sediments. Meanwhile, the eastern terrane (mainly Loei-Phetchabun fold belt) had not been uplifted probably until, the accumulation of the Phu Khat Formation terminated. Subsequently, the whole region began to uplift forming a high mountainous area since the Ypresian time when the Greater India collided with the Eurasia.
The petroleum exploration has been conducted in the Khorat Plateau since 1962 and two gas fields have been discovered and commercially produced. The lacustrine facies of the Huai Hin Lat Formation is believed to be one of the main source rocks of the gas. Therefore, investigation and analysis of the Huai Hin Lat shale for understanding the paleoenvironment and petroleum source rock are carried out in this study. Petrographical study and geochemical analysis of shale samples were performed to explain the paleoproductivity and past redox condition. The palynofacies assemblage comprises abundant AOM, acritarchs, phytoclasts, and very small amount of spores and pollen. Geochemical analysis was used to determine the total organic carbon (TOC) and the concentration of major, trace, and rare earth elements. The paleoproductivity proxies are composed of palynofacies, TOC, excess SiO2, Ba/Al, and P/Al. They reflect a high paleoproductivity except the middle of the lower part (bed 3) and the lower bed 13 of the upper part. Bed 3 shows the highest peak of TOC and the lower bed 13 exhibits a relatively lower TOC, which can be explained by the excellent and the poorer preservation condition, respectively. The paleoredox proxies consist of U/Th, V/Cr, NiCo, (Cu+Mo)/Zn, Ni/V, and Ce anomaly. They are used to establish the depositional environments, to characterize the organic matter content, and to assess the source rock potential. They reflect many high peaks and predominantly high values of paleoredox proxies except the middle part and the lower bed (lower bed 13) of the upper part. They indicate that the section was mainly under anoxic or reducing condition, which is supported by the high Ce/Ce* (> 0.8) and V/Cr (> 2.0) values. The middle of the lower part (bed 3) shows lower productivity but it contains the highest peak of TOC, which is conformed to be the excellent preservation of organic matters in the strong reducing condition. The middle part, which shows high productivity, contains relatively lower TOC as it possesses a less reducing condition compared to the more reducing intervals. The lower bed 13 of the upper part shows a less reducing condition and a lower TOC, which conforms to a lower productivity. The organic matters of the Huai Hin Lat Formation consist mainly of AOM and acritarchs and possess good to excellent TOC (2%–7%). They belong mainly to type I and type II kerogens with some mixture of type III as indicated by the presence of phytoclasts, spores, and pollen. The organic matters of the Huai Hin Lat Formation, based on the kerogen type and the thermal history, have already generated significant amount of oil and some gas to the Sap Phlu Basin.
The volcanic rocks from the Sayabouli area in northwestern Laos have been poorly studied. These volcanic rocks are traditionally mapped as the Permian–Early Triassic sequences on the geological map. One basaltic-andesite from the Sayabouli area yields a zircon U-Pb age of 237.7±1.7 Ma, suggesting a Middle Triassic origin. All basalt and basaltic-andesite samples from the Sayabouli area show depletions in HFSEs (e.g., Nb, Ta, Ti) and have high LILE/HFSE ratios, and exhibit the geochemical affinity to the continental arc volcanic rocks and are geochemically similar to the continental arc volcanic rocks from the Phetchabun belt in northeastern Thailand, suggesting a Late Permian–Middle Triassic continental margin in the Sayabouli area of northwestern Laos and Phetchabun area of northeastern Thailand. Our data indicate that the Phetchabun arc volcanic belt through the western Loei sub-belt can be linked to the Sayabouli area in northwestern Laos.
The Nan-Uttaradit suture is marked by a narrow N-S trending and discontinuous ophiolite belt in northern Thailand. This suture zone is a mélange composed of gabbro, tholeiitic metabasalt, andesite and radiolarian chert. Samples of gabbro and meta-basalt in the Nan-Uttaradit suture yield zircon U-Pb ages of 311±10 and 316±3 Ma, respectively, interpreted as the crystallization ages of the rocks, suggesting the Nan-Uttaradit Ocean existed in the Late Carboniferous. Our results indicate that the Nan-Uttaradit Ocean co-existed with the Ailaoshan-Jinshajiang Ocean to the north and was probably an along-strike extension of the latter.
Zircon U-Pb dating, Lu-Hf isotopic and geochemical data for the Silurian rhyolites from the Loei fold belt are presented to constrain their petrogenesis and tectonic settings. The rhyolites give a weighted mean 206Pb/238U age of 423.7±2.7 Ma, and are characterized by high SiO2, Al2O3, K2O and low MnO, MgO and P2O5. All samples are enriched in LILEs (e.g., Ba, K, Pb) and LREEs and depleted in HFSEs (e.g., Nb, Ta, Ti) with obvious negative Eu-anomalies (dEu=0.56-0.63). The calc-alkaline rhyolites are typical arc-related rocks. The Loei rhyolites have high A/CNK ratios (1.19-1.34) and positive εHf(t) (4.03-5.38), which can be interpreted as partial melting of juvenile crustal materials followed by multistage melting and differentiation, similar to highly fractional I-type rocks. Combined with regional geological surveys, the Loei rhyolites should be formed in a volcanic arc environment and may be in contact with the Truong Son fold belt during the Early Paleozoic. Moreover, the Simao Block might be in contiguity with the Indochina Block during Silurian.
The Changning-Menglian belt, distributed over southwestern Yunnan Province in Southwest China, contains oceanic rocks that are considered to be remnants of the Paleotethys. This study observed Triassic siliceous rocks of the Muyinhe Formation in the Changning-Menglian belt and analyzed their geochemistry. The samples have high concentrations of SiO2 (81.65 wt.%–88.38 wt.%; average: 84.99 wt.%±2.14 wt.%). Most of the samples were plotted in the non-hydrothermal field on the Al-Fe-Mn diagram. Most of the samples were plotted in the continental margin field on the Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3) and (La/Ce)N-Al2O3/(Al2O3+Fe2O3) diagrams. Moreover, the samples show a flat REE (rare earth element) pattern normalized to NASC (North America shale composite). These geochemical results, in addition to the lack of rhythmical bedding of the siliceous rocks, strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits. Although previous studies have suggested that the siliceous rocks are pelagic deposits, the present results indicate that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.
The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ~259 Ma, and four subordinary age-peaks at ~1 859, ~941, ~788, and ~447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.
The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand (the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone). Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high FeOt (20.96 wt.%–25.56 wt.%) and TiO2 (6.07 wt.%–6.34 wt.%) and low SiO2 (38.54 wt.%–43.46 wt.%) and MgO (1.61 wt.%–2.40 wt.%) contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the "normal" Bowen trend. Their chondritenormalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial 87Sr/86Sr ratios and εNd(t) values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB (oceanic-island basalt). These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.
The Ailaoshan-Red River (ASRR) shear zone in SW China represents an important discontinuity believed to have accommodated eastward extrusion of the Tibetan Plateau in response to the collision of the Indian and Eurasian plates. The onset timing and duration of the ASRR sinistral strike-slip shearing have been hotly disputed. In this paper we present new zircon LA-ICP-MS U-Pb geochronological data from six syntectonic granitic mylonite and leucosomes samples from the ASRR shear zone. Our data reveal a metamorphic age of ~40 Ma, most likely suggesting the maximum age of the shearing initiation. Rocks showing syn-kinematic signatures yield crystallization ages of 38–22 Ma, with inherited components ranging from 716 to 108 Ma. These results, together with existing geological and geochronological data, indicate that the sinistral shearing along the ASRR zone probably began at 40 Ma, mainly activated at 29–22 Ma and lasted at least to ~22 Ma. Our data suggest a continuous extrusion between the Indochina and South China blocks during ~35–17 Ma. The ASRR sinistral shearing has accommodated large scale eastward displacement of the southeastern Tibetan syntaxis, and is likely responsible for the opening of the South China Sea.
The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166-161 and 139±2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76×10-6-249.71×10-6), Rb/Sr ratios (1.19-58.93), pronounced Eu negative anomaly (0.01-0.37) and low Nb/Ta ratios (2.40-6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35×10-6- 282.17×10-6), Rb/Sr ratios (2.05-10.30) and relatively high Nb/Ta ratios (4.45-13.00). The isotopic data of the MTS granites display relatively enriched values, with ISr varying from 0.708 2 to 0.709 7, εNd(t) from -7.8 to -6.9 and εHf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.703 05-0.704 77, εNd(t)=-5- -3.4 and εHf(t)=-0.7-1.8). The two-stage model ages of the MTS granites (T2DM(Nd)=1.51-1.59 Ga and T2DM(Hf)=1.26-1.48 Ga) are also higher than those of the HLT granites (T2DM(Nd)=1.21-1.34 Ga and T2DM(Hf)=0.96-1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.
This paper reports geochronological data of detrital zircons from the country rock and sedimentary xenoliths of the Cilincuo pluton (79±0.7 Ma) in the southern Yidun arc belt and the inherited zircons from the Late Triassic granites in the eastern Yidun arc belt, eastern Tibet Plateau. Detrital zircons ages from the sedimentary xenoliths have four prominent peaks at 2.5–2.4 Ga, 1.9–1.8 Ga, 480–400 Ma, and 350–300 Ma, whereas those from the country rock exhibit another four prominent peaks at 1.9–1.8 Ga, 850–700 Ma, 480–400 Ma, and 300–250 Ma. Based on comparison with age data from previous studies, we suggest that the sedimentary xenoliths are from the Lanashan Formation and the major provenance of them is Qiangtang Block, Zhongza massif and South China Block, whereas the country rock belongs to the Lamaya Formation and the major provenance of them is similar to those of the neighbouring Songpan-Garzê terrane. In addition, the inherited zircons from the Late Triassic granites in the eastern Yidun arc belts have a prominent Neoproterozoic age population (900–700 Ma), which suggests that there is an old basement with west Yangtze Craton affinity beneath the Triassic sediments. Combining with previous studies, we propose that the provenances of the formations vary from the Lanashan Formation to the Lamaya Formation which may indicate a record of the final closure of the Garzê-Litang Ocean.
This study reports zircon U-Pb and Hf isotopes and whole-rock elemental data for granodiorites from the East Kunlun orogen. The zircon U-Pb dating defines their crystallization age of 235 Ma. The rocks are characterized by high-K calc-alkaline, magnesian and metaluminous with (K2O+Na2O)=6.38 wt.%–7.01 wt.%, Mg#=42–50 [Mg#=100×molar Mg/(Mg+FeOT)], A/CNK=0.92–0.98, coupled with highεHf(t) values from -0.65 to -1.80. The rocks were derived from partial melting of a juvenile mafic crustal source within normal crust thickness. The juvenile lower crust was generated by mixing lithospheric mantle-derived melt (55%–60%) and supracrustal melt (40%–45%) during the seafloor subduction. Together with available data from the East Kunlun, it is proposed that the studied Middle Triassic granodiorites were formed in post-collisional extension setting, in which melting of the juvenile lower crust in response to the basaltic magma underplating resulted in the production of high-K granodioritic melts.
The multi-stage geological evolution and extensive continental deformations during the course of its history make the Central Asian metallogenic region (CAMR) a unique and complicated large-scale metal domain. New geological observations and precise age-data allow an improved reconstruction of the geological evolution of the CAMR. This paper summarizes the Paleozoic orogenic evolution and related ore formation in the core part of the CAMR based on the geological data published both during the Soviet period and the last decades. Four ore-formation provinces (Altay, Balkhash-Junggar, Chu-Yili-Tianshan, and Southwest Tianshan) could be classified. The Balkhash-Junggar and Chu-Yili-Tianshan provinces are the major topics of this paper. The Balkhash-Junggar province consists of 4 huge ore-forming belts (Zharma-Saur, Tarbahtay-Xiemistay, Aktogay-Baerluke, Balkhashwestern Junggar) with 11 large ore-college areas. The Chu-Yili-Tianshan province consists of 4 huge ore-forming belts (Alatau-Sairimu, Chu-Yili-Bolehuole, Issyk-Awulale, Kazharman-Nalaty) with 22 large ore-college areas. Formation of large ore-college area corresponds to a specific stage of continental crust growth. Comparison of geology and ore deposits in the CAMR provides rich information for future exploration and understanding of ore-forming processes. The Paleo-Junggar Ocean closed at Early Devonian in the Balkhash-western Junggar ore-forming belt. Afterwards, widespread volcanicsedimentary rocks formed at extensional stage due to delamination of the thick lower crust formed during previous accretionary processes. Felsic magma intrusion caused formation of porphyry Cu-Au deposit at ~310 Ma and related hydrothermal gold deposits about 10 Ma later. For example, in the Hatu-Baobei-Sartohay Au-Cr ore-college area in the Balkhash-western Junggar ore-forming belt, small granitic to diorite plutons and various dykes (312–277 Ma) and large granite bodies (~300 Ma) intruded into the Devonian to Early Carboniferous volcano-sedimentary basin. These magmatic activities and fault systems mainly controlled ore-forming processes.
Shahre-Babak ophiolite is a part of the inner Zagros ophiolite belt in Iran. Major parts of intrusive masses of Share-Babak ophiolite are gabbro and plagiogranite. The SiO2 versus Na2O+K2O diagram shows that the palgiogranites are related to calk-alkaline series. Rare earth elements exhibit relatively similar pattern that indicates these rocks are syngenetic. Also, REE patterns display an enrichment of LREE compared to HREE, and are characterized by flat to slightly concaveup patterns from Gd to Yb. Such patterns contrast sharply with those of plagiogranites in more complete ophiolite sequences, such as the Semail ophiolite, Oman, or the Troodos ophiolite, Cyprus, and Neyriz, where patterns are much flatter and slightly LREE-depleted. The slightly LREE-enriched patterns of the Shahre-Babak plagiogranites support a partial melting origin for them. The low TiO2, Nb, Ta content and high LREE concentrations of the Shahre-Babak plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites, which were related to hydrothermal alteration of gabbros in intra-oceanic back-arc basin.
The ultramafic dikes in the Tarim large igneous province (Tarim LIP), exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo (Mg/(Mg+Fe)) numbers ranging from 73 to 85, which are not in equilibrium with the olivine (Mg# of 89) from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid (parental magma) composition of SiO2 of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%–18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al2O3 of 8.70 wt.%–11.62 wt.% and TiO2 of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike (ca. 45%–60% in the whole rock). The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 ºC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.