Thermo-tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74-90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K-Ar ages and the Turonian-Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100-80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5-3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene-Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific-Eurasian plate and the demise of the Kula-Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.     

南北地震带应力场演变的数值模拟研究

应用有限元方法,考虑主要地质构造情况,就粘弹介质状态下南北地震带及其相邻区域的应力场时空演变过程进行了数值模拟研究．计算了10万零10年累积载荷作用下模型的应力场演变,发现在周边板块不断推挤作用下,当整个南北带的应力松弛趋于稳定后,带上应力场的总体扰动大小取决于印度-澳大利亚板块、菲律宾板块和太平洋板块的共同作用．南北地震带上呈现出的“地震南北呼应”现象,就是区域应力场达到平衡态后,整个南北带对外载荷(板块边界力)作用的总体反应．     

Colors and collision rates within the Kuiper belt: problems with the collisional resurfacing scenario

We present a numerical check of the collisional resurfacing (CR) hypothesis proposed to explain the observed color diversity within the Kuiper Belt (where surface reddening due to space weathering is counteracted by regular resurfacing of neutral material after mutual collisions). Deterministic simulations are performed in order to estimate the relative spatial distribution of kinetic energy received by collisions, , for a population of target Kuiper Belt objects (KBOs) embedded in a swarm of impactors distributed within the belt. Four different impactor disks have been considered, depending on the excitation and the external limit of the belt and the density of the scattered KBOs (SKBOs) population. The obtained results are compared to the relative color index distribution within the observed Kuiper Belt, in order to derive possible similarities between the high vs low objects spatial distribution in our simulations and the bluer vs redder KBOs distribution in the “real” Kuiper Belt. Such similarities are found for several important features, in particular the general correlations between highly impacted objects and high rms excitation and low perihelion q values that are in good agreement with equivalent correlations found for the bluest objects of the observed belt. Nevertheless, simulations disagree with observations on two crucial points. (1) The plutinos are significantly more collisionally affected than the rest of our test KBO population, whereas there is no observed tendency toward bluer plutinos. (2) There is always a much stronger correlation between and eccentricities than inclinations, whereas observations show just the opposite feature. The presence of numerous SKBO impactors could significantly damp these problematic features, but cannot erase them. Whether these contradictions invalidate the whole CR scenario or not remains yet uncertain, since the physical processes at play are still far from being fully understood and the sample of available observational data is still relatively limited. But it seems nevertheless that the scenario might not hold in its simple present form.     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.     

Petrological and geochronological constraints on the origin of the Palimé–Amlamé granitoids (South Togo, West Africa): A segment of the West African Craton Paleoproterozoic margin reactivated during the Pan-African collision

The Palimé–Amlamé Pluton (PAP) in southern Togo, consists of silica-rich to intermediate granitoids including enclaves of mafic igneous rocks and of gneisses. They are commonly called the “anatectic complex of Palimé–Amlamé” and without any convincing data, they were interpreted either as synkinematic Pan-African granitoids or as reworked pre Pan-African plutons. New field and petrological observations, mineral and whole-rock chemical analyses together with U–Pb zircon dating, have been performed to evaluate the geodynamic significance of the PAP within the Pan-African orogenic belt. With regard to these new data, the granitoids and related enclaves probably result from mixing and mingling processes between mafic and silicic magmas from respectively mantle and lower crust sources. They display Mg–calc-alkaline chemical features and present some similarities with Late Archaean granites such as transitional (K-rich) TTGs and sanukitoids.

The 2127 ± 2 Ma age obtained from a precise U/Pb concordia on zircon, points out a Paleoproterozoic age for the magma crystallization and a lower intercept at 625 ± 29 Ma interpreted as rejuvenation during Pan-African tectonics and metamorphism. Based on these results, a Pan-African syn to late orogenic setting for the PAP, i.e. the so-called “anatectic complex of Palimé–Amlamé”, can be definitively ruled out. Moreover according to its location within the nappe pile and its relationships with the suture zone, the PAP probably represents a fragment of the West African Craton reactivated during the Pan-African collision.     

Linking the Sulu UHP belt to the Korean Peninsula: Evidence from eclogite, Precambrian basement, and Paleozoic sedimentary basins

Considerable debate on whether and how the Sulu Orogenic Belt extends eastward to the Korean Peninsula has remained over the past decade. New results reported here include the following: (1) an eclogite and retrograded eclogite-bearing complex (Hongseong Complex) is discovered in South Korea, in which the eclogite occurs as lenses in circa  810–820 Ma granitic gneiss. SHRIMP zircon dating of the eclogite yields  230 Ma for the metamorphic age and  880 Ma for the protolith age; (2) The basement of the Rangnim, Gyeonggi and Yeongnam massifs have affinities to the basement of the North China Block (NCB). However the Gyeonggi Massif encloses a minor amount of large or small slabs of the Hongseong Complex that are similar to the rocks of the Sulu Belt. (3) Two main Paleozoic basins within the Rangnim and Gyeonggi massifs have a similar Paleozoic tectono-stratigraphy to the NCB. (4) The Imjingang and Ogcheon belts do not exhibit any metamorphic characteristics of collisional orogenic belts. Based on these facts, we propose a crustal-detachment and thrust model and suggest that the collision belt between the Yangtze Block (YB) and NCB (Sino–Korea Craton) is preserved along the western margin of the Korean Peninsula. The lower part of the UHP metamorphosed lithosphere of the YB was subducted under the Korean Peninsula and not uplifted to the surface. The lower crust of the YB (the Hongseong Complex) was detached from the subducted lithosphere and thrust over the Korean Peninsula, and inserted into the basement rocks of the Gyeonggi Massif. The upper crust of the YB possibly was detached from the lower crust and overthrusted along the Honam and Chugaryong shear zones. The Imjingang and Ogcheon belts possibly represent the detached upper crust of YB and their present occurrences are controlled by a Mesozoic strike–slip shear structure. All these detached lower and upper crustal slabs were strongly deformed during the Late Jurassic and Early Cretaceous tectonic event leading to their present geological distribution and characteristics.     

The Palaeozoic tectono-metallogenic evolution of the northern Tasman Fold Belt System, Australia: Interplay of subduction rollback and accretion

The Tasman Fold Belt System in eastern Australia provides a record of the Palaeozoic geological history and growth of the Australian continent along the proto-Pacific margin of Gondwana inboard of an extensive and long-lived subduction system. The Hodgkinson and Broken River provinces represent prominent geological elements of this system and together form the northern Tasman Fold Belt System. Geochronological age dating of the timing of gold formation in the Amanda Bel Goldfield in the Broken River Province and the Hodgkinson Goldfield in the Hodgkinson Province provides constraints on the occurrence of a deformation and mineralisation episode in the Late Devonian–Early Carboniferous. Integration of these newly-obtained data with petrogenetic constraints and a time–space evaluation of the geological evolution of the Hodgkinson and Broken River provinces, as well as other terranes in the northern Tasman Fold Belt System, allows for the development of a geodynamic model for the Palaeozoic evolution of the northern Tasman Fold Belt System. Our model indicates that three cycles of extension–contraction occurred during the Palaeozoic evolution of the northern Tasman Fold Belt System. Episodes of extension were controlled by rollback of the subduction system along the proto-Pacific margin of Gondwana, whereas episodes of contraction resulted from accretion following the arrival of positively buoyant segments (i.e., micro-continental blocks/oceanic plateaus) at the subducting trench.Our composite interpretative model on the geodynamic evolution of the northern Tasman Fold Belt System integrates the timing of the development of mineral deposits throughout this part of the system and provides a significant advancement in the understanding of Palaeozoic geodynamics along the margin of Gondwana in northeast Australia and allows comparison with the southern part of the Tasman Fold Belt System.     

The Temaguessine Fe-cordierite orbicular granite (Central Hoggar, Algeria): U–Pb SHRIMP age, petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe₂O₃ for 72% SiO₂). Strongly negative ε_Nd (−9.6 to −11.2), Nd T_DM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high I_Sr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.     

Prediction of acid mine drainage generation potential in selected mines in the Ashanti Metallogenic Belt using static geochemical methods

Acid mine/rock drainage (AMD/ARD) is the biggest environmental threat facing the mining industry. This study investigates AMD/ARD possibilities in three mines in the Ashanti Belt, using acid base accounting (ABA) and net acid generation pH (NAGpH) tests. Twenty-eight samples of rock units and mine spoil from these mines were collected for ABA and NAGpH tests. Two tailing dumps at Prestea and Nsuta were confirmed by both methods as acid generating with NAGpH of 4.5 and 4.6 and neutralization potential ratio values of 4.38 and 4.60, respectively. Six other samples are classified as potentially acid generating using a variety of established classification criteria. The rest of the samples either exhibited very low sulphur and carbonate content or had excess carbonate over sulphur. Consistency between results from ABA and NAGpH tests validates these tests as adequate tools for preliminary evaluation of AMD/ARD possibilities in any mining project in the Ashanti Belt.     

辽北地区早石炭世变质火山岩年代学和地球化学特征:对华北板块北缘东段构造演化的启示

华北板块北缘东段分布的构造混杂岩带为研究古亚洲洋的演化提供了重要的依据,"下二台岩群"作为该构造混杂岩带的重要组成部分,其形成时代和构造属性仍存在争议。详细的研究表明下二台地区变质火山岩原岩包括流纹岩、英安岩、安山岩,为一套钙碱性火山岩,属于准铝质-弱过铝质岩石,根据岩相学和地球化学特征将其分为变质酸性火山岩和变质中性火山岩;二者均相对富集轻稀土元素,亏损重稀土元素,轻重稀土元素分馏明显,Eu负异常不明显,但变质中性火山岩稀土总量低于变质酸性火山岩,变质酸性火山岩明显亏损Sr、P元素,结合野外产出面积和高场强元素相关性特征,认为二者不是同一基性岩浆分异的产物。变质火山岩锆石LA-ICP-MS U-Pb年龄为341～348Ma,代表其原岩结晶年龄。变质酸性火山岩原始岩浆来自于地壳物质的部分熔融,变质中性火山岩原始岩浆来自于俯冲带附近岩石圈地幔,并遭受了地壳物质的混染,二者均形成于活动大陆边缘火山弧环境。最新研究成果表明"下二台岩群"由不同时代、不同构造环境下形成的地质单元叠置混杂而成,称其为"下二台"构造杂岩更为准确。下二台地区变质火山岩表明在早石炭世初,古亚洲洋板块已经南向俯冲,在华北板块北缘形成活动大陆边缘弧环境,早石炭世变质火山岩原岩为这一俯冲阶段的产物。