Hegenshan Ophiolite Research Articles

Mantle property of Hegenshan ophiolite in Inner Mongolia: Constrains from element geochemistry and Re-Os isotopes of harzburgite

贺根山蛇绿岩带是中亚造山带东部出露面积最大的晚古生代蛇绿岩带, 然而研究者对它是大洋岩石圈的残余还是陆内岩石圈地幔伸展的产物仍存在较大争议, 主要归因于一直缺少橄榄岩Re-Os同位素的有效制约。基于此, 本文对贺根山蛇绿岩中方辉橄榄岩进行了系统的岩石学、地球化学和Re-Os同位素研究。方辉橄榄岩主要由橄榄石、斜方辉石、粗粒的铬尖晶石和少量单斜辉石组成。贺根山方辉橄榄岩具有难熔地球化学特征, 显示出高的MgO(45.1%~48.1%)含量, 低的CaO(0.09%~0.26%)和Al₂O₃(0.58%~0.90%)含量及Tb/Yb比值, 低于原始上地幔的¹⁸⁷Os/¹⁸⁸Os(0.1236~0.1263)同位素组成; 橄榄石具有较高的Fo值(91.0~91.7)和NiO含量(0.26%~0.48%), 而且单斜辉石具有低的重稀土含量, 暗示了贺根山方辉橄榄岩是尖晶石相陆下岩石圈地幔高程度(20%~30%)部分熔融的难熔残余。另外, 方辉橄榄岩相对富集LREE和大离子亲石元素, 亏损高场强元素(例如Nb、Ta), 且单斜辉石具有高Ti/Eu值和低的La/Yb, 揭示了地幔源区受到硅酸盐熔体交代作用的改造。另一方面, 方辉橄榄岩显示出相对年轻的Re亏损模式年龄, <i>t</i>_RD为454~825Ma, 表明贺根山地区岩石圈地幔形成的时代应该在0.8Ga之前, 而后经历过多期熔体抽离和交代, 与区域上大规模地壳增生时代吻合, 可能是罗迪尼亚超大陆裂解的响应。结合蛇绿岩中大量深源矿物(金刚石等)和捕获锆石的存在、南北两侧一致的地质记录, 本文认为贺根山方辉橄榄岩可能是陆内伸展环境下深部地幔物质上涌减压熔融的产物。

Carboniferous–Early Permian sedimentary rocks from the north‐eastern Erenhot, North China: Implications on the tectono‐sedimentary evolution of the south‐eastern Central Asian Orogenic Belt

Reconstructing the Late Palaeozoic tectonic process of the Hegenshan Ophiolite Belt is critical for evaluating the protracted accretion and collision history of the south‐eastern Central Asian Orogenic Belt. The Carboniferous–Permian sedimentary rocks near the Erenhot area are key to revealing the evolutionary history of the Hegenshan Ocean. In this study, new stratigraphic, geochemical, and detrital zircon U–Pb age analyses were conducted on three Carboniferous–Early Permian strata (viz., Halatumiao, Amushan, and Benbatu formations). Deposition of the Halatumiao Formation began around 313 Ma, inferred from a zircon U–Pb age from a lower tuff layer. The end of deposition is constrained by two later intrusive high‐K granite plutons that give ages of 277–285 Ma. The predominant detrital zircon group in these strata is characterized by ages of 340–290 Ma, high positive εHf(t) values, and juvenile TDM2 ages, which suggest adjacent and unitary juvenile provenances. The Halatumiao Formation mainly consists of blackish fine‐grained sedimentary rocks with a thickness of 6–7 km implying a high sedimentation rate and a stable hydrodynamic condition. Thus, the Halatumiao Formation most likely indicates the existence of a deep and wide ocean basin under an effect of long‐lived and continuous extension. The Amushan Formation has a comparable depositional age to the Halatumiao Formation, based on three detrital zircon Palaeozoic age populations with peaks of 317, 389, and 441 Ma, but contains more complex provenances. The Benbatu Formation has an earlier deposition age between ca. 324 and 314 Ma. Its detrital zircons yield a predominant age group at around 410–440 Ma but lack syn‐sedimentary Late Palaeozoic age. Unlike the Halatumiao Formation, the two other formations are considerably smaller in terms of their deposition thickness and outcrop area. The transition from Benbatu to Amushan and Halatumiao formations records the development of a gradually deeper and wider ocean basin in response to a back‐arc extensional setting from the Late Carboniferous to the Early Permian.

Harzburgite found in the Hegenshan ophiolite, southeastern Central Asian Orogenic Belt: Petrogenesis and geological implications

This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic mélange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al2O3 contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al2O3 and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T–fO2 conditions support a back-arc setting for the Hegenshan ophiolitic mélange.

Termination of the Hegenshan Orogen in the Xing'an–Mongolian Orogenic Belt, North China: Geochemical and zircon U–Pb geochronological constraints from Early Permian mafic dykes

Based mainly on field geological observation, we identified the Dong Ujimqin mafic dykes in Inner Mongolia, north of Hegenshan ophiolite accretionary belt. The dykes are mainly dolerite, and their ages and source were determined using geochronological, geochemical, zircon Hf, and whole‐rock Sr–Nd isotopic data. Photomicrographs reveal ophitic texture and amygdale structures of the dolerite dykes. Laser ablation multiple collector inductively coupled plasma mass spectrometry U–Pb analysis of zircons yielded ages of 277.9 ± 1.8 and 275.9 ± 1.2 Ma for two of these mafic dykes, placing them within the Early Permian. The dykes are classified as alkaline and tholeiitic series according to their major elements and characterized by high Na2O (2.62–4.06 wt.%), high TiO2 (1.63–2.71 wt.%), and low K2O (0.74–2.06 wt.%). The mafic dykes have similar geochemical characteristics that exhibit large‐ion lithophile element enrichment relative to high‐field‐strength elements, depletion in Ta and Nb, and an asthenosphere‐like Sr–Nd–Hf isotopic signature. They have high Sm/Nd (0.131–0.141) and LaN/YbN (3.75–9.17), near‐chondritic heavy rare earth elements ([Gd/Yb]N = 1.21–2.39), and low Rb/Sr (0.03–0.08) ratios with negligible Eu anomalies (Eu/Eu* = 0.85–1.00). The dykes contain relatively low amounts of radiogenic Sr, as shown by (87Sr/86Sr)i values (0.7035–0.7042), and have positive values of εNd(t; 4.31–6.96) and εHf (t; 6.0–14.5) and negative values of εSr(t; −0.28 to −10.31), suggesting that the mafic dykes have been derived from partial melts of the depleted lithospheric mantle contaminated by crust in the source region and formed in post‐orogenic settings. All the above indicate an extensional event, which was related to the post‐subduction of the Paleo‐Asian Ocean, at ~277–275 Ma, and significant vertical continental growth of the crustal continuum occurred in the continental interior during post‐collisional extension.

Ages and Hf isotopes of detrital zircons from the Permian strata in the Bengbatu area (Inner Mongolia) and tectonic implications

The Central Asian Orogenic Belt (CAOB) was built up through protracted accretion and collision of a variety of terranes/micro-continents during Neoproterozoic–Mesozoic time. To understand potential links among Paleozoic subduction and accretionary processes that were operative during the development of the southeastern CAOB, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from previously defined Devonian, Carboniferous and Early Permian strata in the Bengbatu area, Inner Mongolia. Detrital zircons from (meta-) sandstones in these strata commonly yield major Paleozoic age populations at ca. 300–261 Ma, 351–300 Ma and 517–419 Ma, and also give several Precambrian ages that range from 2687 Ma to 544 Ma. The youngest ages redefine the deposition of all these strata to be in the Middle Permian (Wordian–Capitanian) or later, much younger than previously considered. These ages, coupled with regional magmatic records, support an interpretation of most surrounding areas as possible detritus sources, including the Mongolian arcs to the north, the Northern Accretionary Orogen to the south, and the intervening Erenhot–Hegenshan Ophiolite Belt. Zircons with magmatic ages of ca. 500–350 Ma and ca. 300–261 Ma display a large range of εHf(t) values (−13.97 to +15.31), whereas ca. 350–300 Ma zircons are dominated by positive εHf(t) values (+0.14 to +16.00). These results support the occurrence of two significant shifts of the zircon εHf(t) values, which has tectonic implications for the understanding of the Carboniferous–Permian evolution of the southeastern CAOB. A marked shift from mixed to positive zircon εHf(t) values at 350–330 Ma likely manifests the incipient opening of the Hegenshan Ocean, due to the slab rollback of the subducting Paleo-Asian Oceanic lithosphere. Another shift from positive to mixed zircon εHf(t) values at ca. 300 Ma likely corresponds to a tectonic switch from syn-orogenic subduction-related to post-orogenic extensional setting, genetically related to the tectonic collapse of a formerly overthickened crust.

U–Pb ages of detrital zircon of the Paleozoic sedimentary rocks: New constraints on the emplacement time of the Hegenshan ophiolite, NE China

The Hegenshan ophiolite in the Solonker-Hegenshan belt is the largest ophiolite in the Central Asian Orogenic Belt (CAOB). Despite its significance in constraining regional tectonic evolution, the emplacement time of the Hegenshan ophiolite is still under debate. In this study, we provide new detrital zircon ages of the Paleozoic sedimentary rocks that unconformably overlie the Wusinihei ophiolite (northeastern part of the Hegenshan ophiolite) to constrain the lower limit emplacement time of the Hegenshan ophiolite. The zircon ages obtained from the Paleozoic sedimentary rocks range from 298±8 to 363±7Ma, and show bimodal distribution at 300–320Ma (peak at 308Ma) and 320–360Ma (peak at 330Ma). The age group of 300–320Ma coincides with the age range of the volcanic rocks of the Late Paleozoic Gegenaobao Formation. The age group of 320–360Ma with a peak at 330Ma may be linked to local mafic–ultramafic rocks of the Hegenshan ophiolite. Accordingly, we suggest that the emplacement time of the Hegenshan ophiolite should have occurred earlier than the deposition of the Gegenaobao Formation, most likely during the time between 308 and 330Ma, instead of the Silurian, Devonian or Mesozoic as previously considered.

Geochronological and Hf isotopic variability of detrital zircons in Paleozoic strata across the accretionary collision zone between the North China craton and Mongolian arcs and tectonic implications

The Central Asian orogenic belt formed by accretion subsequent to the contraction of the paleo–Asian Ocean, and its southeastern segment terminated along the Solonker suture zone, amalgamating the Mongolian arcs and the North China craton by the end of the Early Triassic. Since typical regional collisional features are absent, its tectonic evolution remains speculative. An investigation into the variability of detrital zircon U-Pb ages and Hf ratios in Permian strata along a southeast-northwest transect from the Mongolian arcs to the North China craton reveals distinct differences: Northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests that in the early Paleozoic (ca. 429 Ma), the paleo–Asian Ocean was consumed along the Uliastai arc and the North China craton, initiating the formation of the northern and southern accretionary orogens, respectively. By the end of the middle Carboniferous, the Mongolian arcs were consolidated after the accretion of the Uliastai arc. In the late Carboniferous (ca. 314 Ma), the Hegenshan back-arc basin opened, detaching the northern accretionary orogen. While subduction continued there, it may have temporarily ceased in the south after the accretion of a microcontinent (ca. 300 Ma). By the end of the Middle Permian (ca. 269 Ma), back-arc basin closure led to the formation and obduction of the suprasubduction zone Hegenshan ophiolite. Eventually, the paleo–Asian Ocean closed by double-sided subduction. Such accretionary wedge-wedge collision would not involve continental deep subduction, unlike other continental collisions. The early stages of the sequence likely resembled a Pacific-type scenario dominated by the subduction of oceanic lithosphere, including a Japanese Sea–type back-arc basin opening. The late stages involved the accretion of large-scale continental blocks, ultimately leading to final ocean closure, which may be similar to the archipelago-type setting of present-day Southeast Asia.

The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite, Inner Mongolia, China

AbstractDiamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein‐like bodies hosted in dunite, clinopyroxene‐bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al‐rich, with Cr# [100Cr/(Cr+Al)] of about 47–53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides (mostly hematite, magnetite, rutile, anatase, cassiterite, and quartz), sulfides (pyrite, marcasite and others), silicates (magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine) and others (e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys). This study demonstrates that diamond, moissanite and other exotic minerals can occur in high‐Al, as well as high‐Cr chromites, and significantly extends the geographic and age range of known diamond‐bearing ophiolites.

The emplacement time of the Hegenshan ophiolite: Constraints from the unconformably overlying Paleozoic strata

Controversy has long surrounded the emplacement time of the Hegenshan ophiolite that is considered to mark a suture zone, called the Hegenshan–Heihe suture, resulting from the closure of a back-arc basin in the Paleo-Asian Ocean. The Hegenshan ophiolite in the Xiaobaliang area is unconformably overlain by a sequence of Paleozoic strata, called the Zhesi Formation that consists of conglomerate, sandstone, siltstone and limestone, some of which contain Permian marine fossils of Brachiopods. Therefore, the ages of these Paleozoic strata can be used to constrain the emplacement time of the Hegenshan ophiolite. Four samples of the Zhesi Formation collected in the Xiaobaliang area yield the detrital zircon U–Pb ages of 285–272Ma (with the peak at 279Ma), 315–288Ma (with a peak at 300Ma), 320–358Ma (with a peak at 336Ma), and 406±3Ma, of which the ~280 and ~300Ma age groups are remarkably similar to the ages of latest Carboniferous–Early Permian Gegenaobao/Dashizai Formation, or A-type granites, which formed under a post-collisional setting. However, the age groups of 320 to 358Ma with a peak at 336Ma, show the features of mafic–ultramafic zircons in CL image, most likely derived from local mafic–ultramafic rocks of the Hegenshan ophiolite in the Xiaobaliang area, which is supported by the fact of the ophiolite unconformably overlain by the Middle Permian Zhesi Formation. Therefore, we propose that the emplacement time of the Hegenshan ophiolite must have happened at some time before the Middle Permian (~280Ma), most likely between 300 and 335Ma, not in the Silurian, Devonian or Mesozoic as previously considered.

Zircon U–Pb geochronology and geochemistry of the Maoliger quartz monzodiorites, Inner Mongolia, China: implications for Carboniferous arc magmatism and closure of the Palaeo-Asian Ocean

Recent research has identified an early to late Carboniferous magmatic arc that extends from Suzuo Qi to Xiwu Qi in Inner Mongolia, China, but the eastern extension of this arc is unknown. Understanding the relationship between this arc and the Hegenshan ophiolite belt and Xilamulun Solonker suture zone is important to our understanding of the tectonic evolution of the late Palaeozoic Palaeo-Asian Ocean. Here, we present new zircon laser ablation–inductively coupled plasma mass spectrometry U–Pb and geochemical data for the Maoliger quartz monzodiorites within the Jalaid Qi area. The Maoliger quartz monzodiorites formed at 329 ± 2 Ma, are low-K and tholeiitic, and have geochemical signatures indicative of formation within a magmatic arc. These rocks are large-ion lithophile element (e.g. Rb, Ba, and Sr)-enriched and high-field-strength element (e.g. Nb and Ta)-depleted. Combined with previously published researches, it is suggested that the quartz monzodiorites within the Jalaid Qi area formed contemporaneously with and are geochemically similar to quartz diorites of the Xiwu Qi area and the Baolidao pluton in the Suzuo Qi area. This indicates that the early to late Carboniferous magmatic arc in this region extends eastward to the Jalaid Qi area. This arc is located in an area parallel to a southerly early Permian magmatic arc, suggesting that the Palaeo-Asian Ocean subduction zone migrated south between the early Carboniferous and early Permian. The new data show that the Palaeo-Asian Ocean closed after the late Carboniferous.

Carboniferous and Cretaceous mafic–ultramafic massifs in Inner Mongolia (China): A SHRIMP zircon and geochemical study of the previously presumed integral “Hegenshan ophiolite”

We present SHRIMP zircon ages and whole-rock geochemical data to identify mafic–ultramafic massifs of early Carboniferous and early Cretaceous ages from northern Inner Mongolia, China. The early Carboniferous massifs (Chongenshan–Xiaobaliang–Wusnihei) comprise predominantly lherzolite and minor gabbroic rocks. Zircons from a microgabbro (εNd (t)=9.8) and a plagiogranite (εNd (t)=9) yielded weighted mean 206Pb/238U ages of 354±7Ma and 333±4Ma, that we interpret as reflecting the time of zircon (and magma) crystallization. A volcanic breccia resting unconformably on the ultramafic rocks (serpentinites) has an eruption age of 300±2Ma and provides a younger minimum time limit for the emplacement of the early Carboniferous mafic–ultramafic massifs. In contrast, the early Cretaceous mafic–ultramafic massifs (Hegenshan–Chaogenshan) consist predominantly of harzburgite. A pegmatoid gabbro and a rodingite (εNd (t)=10.6) from the Hegenshan massif have magmatic zircon ages of 139±2Ma and 132±7Ma respectively. The Chaogenshan massif is concentrically zoned with an ultramafic core (dominantly harzburgite–dunite+subordinate lherzolite) surrounded by gabbro (with minor hornblendite) and an outermost volcano-sedimentary sequence (metabasaltic flows+metapelites). We dated zircons from a leuocogabbro (125±2Ma; εNd (t)=10.6), a coarse-grained gabbro (142±4Ma), and a metabasalt (134±2Ma). Zircon xenocrysts (ca. 152–492Ma) are ubiquitous in these and other associated rocks (a coarse-grained gabbro and a diabase), as in the Hegenshan rocks (ca. 152–468Ma). The youngest detrital zircon in outer zone garnet schist (i.e. metapelite) has an age of 132±2Ma. Accordingly, we conclude that the northern Inner Mongolia lherzolite-dominant and harzburgite-dominant massifs are temporally and genetically unrelated. Their emplacement is attributed to asthenospheric upwelling and consequent lithospheric extension during periods of orogenic quiescence in the southern Central Asian Orogenic Belt.

Geochronology and geochemistry of the Hegenshan ophiolitic complex: Implications for late-stage tectonic evolution of the Inner Mongolia-Daxinganling Orogenic Belt, China

The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive ε Nd( t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures ( ε Nd( t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd–Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high ε Nd( t) values (+6.3 to +6.8) and low I Sr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic.

Occurrences of Permian ra-diolarians in central and east-ern Nei Mongol (Inner Mon-golia) and their geological sig-nificance to the Northern China Orogen

The Zhesi (Jisu) Formation of the Middle Permian in Nei Mongol (Inner Mongolia) was commonly considered to be a shallow marine sequence. Here I report the radiolarians found in the argillite bed of that formation in Zhesi and Xilinhot areas. This fact indicates a deep marine sedimentary facies persisted during the Middle Permian, and suggests that the ocean between the North China Block and Siberian Craton was not closed until the Late Guadalupian. The suture of this two blocks is probably extends along the Linxi ophiolite belt, south of the Hegenshan ophiolite belt.

Comments on “Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China” by T. Nozaka and Y. Liu [Earth Planet. Sci. Lett. 202 (2002) 89–104

Comments on “Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China” by T. Nozaka and Y. Liu [Earth Planet. Sci. Lett. 202 (2002) 89–104

Reply to Comment on “Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China” by M.-F. Zhou, H.-F. Zhang, P.T. Robinson and J. Malpas

Reply to Comment on “Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China” by M.-F. Zhou, H.-F. Zhang, P.T. Robinson and J. Malpas

Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China

Petrographic observations, and mineralogical and geochemical analyses, have revealed that the Hegenshan ophiolite is of mid-ocean ridge origin and has been subjected to dynamothermal metamorphism at medium P/T conditions. The metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote–amphibolite facies, with peak temperature conditions of 570–640°C at pressures of 4–10 kbar. The amphiboles formed by this metamorphism show K–Ar ages of 110–130 Ma. The metamorphic conditions and K–Ar ages suggest that the Hegenshan ophiolite is located at the suture between the Siberian and North China continental blocks, where the continental collision in this area took place in middle Mesozoic time. Given the temporal and spatial distribution of the igneous activity around the Da Hinggan Ling Mountains, it is suggested that the extensive Yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision. Alternatively, it may possibly have resulted from the collision itself, at the final stage.

Geo–tectonic Position of Tin Polymetallic Mineralization Zone in the Southern Da Hinggan Mountains Area, Inner Mongolia, China: An Introduction to This Special Issue

Abstract: As a part of the main activities of Japan‐China technical cooperation project, a test survey area, approximately 5,000 km2, was established for the implement of its geological and geochemical research program. A major mineralization zone called Huanggang–Ganzhuermiao–Wulanhaote Sn‐Cu polymetallic mineralization zone is recognized in the southern Da Hinggan Mountains area. The southern half of this zone is known as the sole Sn‐mineralization zone in North China. The survey area lies in this prominent zone. As the most of the papers presented in this issue have concerns to the geology and mineralization in this survey area, this report was prepared to introduce geo‐tectonic situation of the Sn‐Cu polymetallic mineralization zone in the Inner Mongolia orogenic belt. The belt is divided into four tectonic facies (from NW to SE); I: Wuliyasitai volcano‐plutonic zone, II: Hegenshan ophiolite mélange zone, III: Sunitezuoqi volcano‐plutonic zone, IV: Wenduermiao ophiolite mélange zone. The subject Sn‐Cu polymetallic mineralization zone is situated in the southeastern part of the Sunitezuoqi magmatic zone. About this Sunitezuoqi magmatic zone, three geo‐tectonic characteristics are pointed out. In late Carboniferous to early Permian period, subduction of Hegenshan oceanic crust occurred, which accelerated volcano‐plutonic activities and brought about basic to intermediate volcanic rocks of tholeiitic to calc‐alkaline series represented by Dashizhai Group in the Sunitezuoqi magmatic zone. Late Jurassic to early Cretaceous acidic rocks representing the most culminated volcanism and plutonism in Mesozoic era in the Da Hinggan Moutains area are distributed very extensively in and around the Sn‐Cu polymetallic mineralization zone. The Proterozoic metamorphic basement rocks called Xilinhaote complex are distributed close to the mineralized area in the Sunitezuoqi magmatic zone.Although the real mineralization was known associated with Mesozoic acidic to intermediate volcano‐plutonic activities, it is thought that the lower Permian Dashizhai volcanic rocks and pre‐Cambrian basement rocks might have played certain significant role in the process respectively of extraction of elements and formation of the magma favorable for such mineralization in the Sunitezuoqi magmatic zone. It would be necessary to give further considerations to these three geological units in relation to the Sn‐Cu polymetallic mineralization.

Geochemical constraints on the origin of the Hegenshan Ophiolite, Inner Mongolia, China

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.