基性麻粒岩造岩矿物微量元素再分配特征及其对变质历史的指示作用:以胶北地体高压麻粒岩为例

高级变质岩的变质历史是反演地壳构造-热事件的重要依据,然而高温扩散和重结晶作用能够改造造岩矿物中的主量元素分布,这对峰期变质温压条件的反演产生很不利的影响。相对于主量元素,微量元素,尤其是离子半径较大的REE,由于其在晶格中的扩散速率远小于主量元素,在高级叠加变质过程有可能记录前期变质作用。本文以胶北地体的高压基性麻粒岩为研究对象,通过详细的岩相学和矿物化学分析,初步解析了变质重结晶过程中的矿物微量元素再分配特征及其对变质作用的指示意义。岩相学上的证据表明这些样品经历了麻粒岩相变质和后期重结晶作用。单矿物的原位化学成分分析,峰期矿物石榴石、单斜辉石的主量元素Mg、Fe、Ca等二价阳离子分布均一,但部分稀土元素及微量元素则表现出钟形剖面环带分布,暗示主量元素遭受到成份扩散及重结晶所致的元素再分配,微量元素可记录峰期历史。结合主、微量元素温压计,我们分别估算了胶东基性高压麻粒岩的峰期(828℃、1.27GPa)和中压麻粒岩相退变质温压条件(810~840℃、0.6~1.0GPa),并推测其后期经历过角闪岩相退变质叠加。结合前人的年代学工作,我们认为该基性麻粒岩经历了近等温快速减压的变质历史。     

中国的板块构造区划

20世纪60年代板块构造学说传入中国,为广大地学工作者所接受并应用于相关的地质工作中。在新一代地质志的研究中,以板块学说为主导,已经成为共识。从板块构造来认识,板块的组成包括其核心及边缘。其核心为克拉通,由稳定的陆块组成;边缘包含了不同的活动大陆边缘和被动大陆边缘。边缘在后期板块汇聚的过程中,常由汇聚或碰撞等不同方式而成为不同的造山带。板块之间具有不同形式的汇聚带,其中主要是地壳对接消减带。在地球发展历史的过程中,不同时期的板块构造格局常有很大的变化。因此,本文以古生代的构造格架为主,探讨对中国大陆进行板块构造区划中的一些相关问题,并对全国的区划提出了一个初步方案。此方案将全国划分为7个I级单元(板块),30个II级单元(克拉通和造山带)以及103个III级单元。     

Upstairs-downstairs: supercontinents and large igneous provinces,are they related?

There is a correlation of global large igneous province (LIP) events with zircon age peaks at 2700, 2500, 2100, 1900, 1750, 1100, and 600 and also probably at 3450, 3000, 2000, and 300 Ma. Power spectral analyses of LIP event distributions suggest important periodicities at 250, 150, 100, 50, and 25 million years with weaker periodicities at 70–80, 45, and 18–20 Ma. The 25 million year periodicity is important only in the last 300 million years. Some LIP events are associated with granite-forming (zircon-producing) events and others are not, and LIP events at 1900 and 600 Ma correlate with peaks in craton collision frequency. LIP age peaks are associated with supercontinent rifting or breakup, but not dispersal, at 2450–2400, 2200, 1380, 1280, 800–750, and ≤200 Ma, and with supercontinent assembly at 1750 and 600 Ma. LIP peaks at 2700 and 2500 Ma and the valley between these peaks span the time of Neoarchaean supercraton assemblies. These observations are consistent with plume generation in the deep mantle operating independently of the supercontinent cycle and being controlled by lower-mantle and core-mantle boundary thermochemical dynamics. Two processes whereby plumes can impact continental assembly and breakup are (1) plumes may rise beneath supercontinents and initiate supercontinent breakup, and (2) plume ascent may increase the frequency of craton collisions and the rate of crustal growth by accelerating subduction.     

The Central Bundelkhand Archaean greenstone complex,Bundelkhand craton,central India: geology,composition, and geochronology of supracrustal rocks

Analysis of 3.3 Ga tonalite–trondhjemite–granodiorite (TTG) series granitoids and greenstone belt assemblages from the Bundelkhand craton in central India reveal that it is a typical Archaean craton. At least two greenstone complexes can be recognized in the Bundelkhand craton, namely the (i) Central Bundelkhand (Babina, Mauranipur belts) and (ii) Southern Bundelkhand (Girar, Madaura belts). The Central Bundelkhand greenstone complex contains three tectonostratigraphic assemblages: (1) metamorphosed basic or metabasic, high-Mg rocks; (2) banded iron formations (BIFs); and (3) felsic volcanics. The first two assemblages are regarded as representing an earlier sequence, which is in tectonic contact with the felsic volcanics. However, the contact between the BIFs and mafic volcanics is also evidently tectonic. Metabasic high-Mg rocks are represented by amphibolites and tremolite-actinolite schists in the Babina greenstone belt and are comparable in composition to tholeiitic basalts-basaltic andesites and komatiites. They are very similar to the metabasic high-Mg rocks of the Mauranipur greenstone belt. Felsic volcanics occur as fine-grained schists with phenocrysts of quartz, albite, and microcline. Felsic volcanics are classified as calc-alkaline dacites, less commonly rhyolites. The chondrite-normalized rare earth element distribution pattern is poorly fractionated (La_N/Lu_N = 11–16) with a small negative Eu anomaly (Eu/Eu* = 0.68–0.85), being characteristic of volcanics formed in a subduction setting. On Rb – Y + Nb, Nb – Y, Rb – Ta + Yb and Ta – Yb discrimination diagrams, the compositions of the volcanics are also consistent with those of felsic rocks formed in subduction settings. SHRIMP-dating of zircon from the felsic volcanics of the Babina belt of the Central Bundelkhand greenstone complex, performed for the first time, has shown that they were erupted in Neoarchaean time (2542 ± 17 Ma). The early sequence of the Babina belt is correlatable with the rocks of the Mauranipur belt, whose age is tentatively estimated as Mesoarchaean. The Central Bundelkhand greenstone complex consists of two (Meso- and Neoarchaean) sequences, which were formed in subduction settings.     

Evolution of continental crust of the Aravalli craton,NW India,during the Neoarchaean–Palaeoproterozoic: evidence from geochemistry of granitoids

Neoarchaean–Palaeoproterozoic granitoids of the Aravalli craton, represented by four plutons with different ages, viz. Gingla (2.6–2.4 Ga), Ahar River (2562 Ma), Untala (2505 Ma), and Berach (2440 Ma) granitoids, are classified into three suites: TTG-like, Sanukitoid, and High-K Granitoid suite, all exhibiting negative Nb and Ti anomalies. The TTG-like suite is characterized by high contents of SiO₂, Na₂O, and LREEs, high (La/Yb)_N, low contents of K₂O, MgO, Cr, and Ni, and low (Dy/Yb)_N, suggesting that this suite formed by partial melting of a subducted basaltic slab without interacting with a mantle wedge. In contrast, the calc-alkaline Sanukitoid suite is marked by a high content of LILEs and mantle-compatible elements, which indicate that this suite formed by partial melting of a slab-fluid metasomatized mantle wedge in a subduction-related arc environment. On the other hand, the High-K Granitoid suite is characterized by high contents of SiO₂ and K₂O, and low contents of Na₂O, MgO, Cr, and Ni with variable Eu anomaly, along with high (La/Sm)_N and (La/Yb)_N, and low (Dy/Yb)_N and Nb/Th. Some high-K granitoids also exhibit A-type characteristics. These features indicate that the High-K Granitoid suite formed by melting of crustal rocks. Early Neoarchaean continental crust formation reflected a slab-melting-dominated magmatic process as evidenced by the TTG-like suite, whereas Palaeoproterozoic petrogenesis was governed by the interaction of slab melt with mantle wedge as demonstrated by the Sanukitoid suite. The High-K Granitoid suite formed during the waning stages of subduction. This study reveals that granitic rocks of the Aravalli craton evolved from slab melting in the Neoarchaean to melting of mantle wedge in the Palaeoproterozoic. Melting of older crust led to the formation of the High-K Granitoid suite.     

Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

The Jericho kimberlites are part of a small Jurassic kimberlitecluster in the northern Slave craton, Canada. A variety of datingtechniques were applied to constrain the nature and age of twoJericho kimberlites, JD-1 (170·2 ± 4·3Ma Rb–Sr phlogopite megacrysts, 172·8 ±0·7 Ma U–Pb eclogite rutile, 178 ± 5 MaU–Pb eclogite zircon lower intercept) and JD-3 (173 ±2 Ma Rb–Sr phlogopite megacryst; 176·6 ±3·2 Ma U–Pb perovskite), and all yielded identicalresults within analytical uncertainty. As there is no discernibledifference in the radiometric ages obtained for these two pipes,the composite Rb–Sr phlogopite megacryst date of 173·1± 1·3 Ma is interpreted as the best estimate forthe emplacement age of both Jericho pipes. The initial Sr isotopecomposition of 0·7053 ± 0·0003 derivedfrom phlogopite megacrysts overlaps the range (0·7043–0·7084)previously reported for Jericho whole-rocks. These strontiumisotope data, combined with the radiogenic initial ²⁰⁶Pb/²⁰⁴Pbratio of 18·99 ± 0·33 obtained in thisstudy, indicate that the Jericho kimberlites are isotopicallysimilar to Group 1 kimberlites as defined in southern Africa.The Jericho kimberlites are an important new source of mantlexenoliths that hold clues to the nature of the Slave cratonsubcontinental mantle. A high proportion (30%) of the Jerichomantle xenolith population consists of various eclogite typesincluding a small number (2–3%) of apatite-, diamond-,kyanite- and zircon-bearing eclogites. The most striking aspectof the Jericho zircon-bearing eclogite xenoliths is their peculiargeochemistry. Reconstructed whole-rock compositions indicatethat they were derived from protoliths with high FeO, Al₂O₃and Na₂O contents, reflected in the high-FeO (22·6–27·5wt %) nature of garnet and the high-Na₂O (8·47–9·44wt %) and high-Al₂O₃ (13·12–14·33 wt %)character of the clinopyroxene. These eclogite whole-rock compositionsare highly enriched in high field strength elements (HFSE) suchas Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934ppm) and Hf (23–64 ppm). This HFSE enrichment is linkedto growth of large (up to 2 mm) zircon and niobian rutile crystals(up to 3 modal %) near the time of eclogite metamorphism. Thediamond-bearing eclogites on the other hand are characterizedby high-MgO (19·6–21·3 wt %) garnet andultralow-Na₂O (0·44–1·50 wt %) clinopyroxene.Paleotemperature estimates indicate that both the zircon- anddiamond-bearing eclogites have similar equilibration temperaturesof 950–1020°C and 990–1030°C, respectively,corresponding to mantle depths of 150–180 km. Integrationof petrographic, whole-rock and mineral geochemistry, geochronologyand isotope tracer techniques indicates that the Jericho zircon-bearingeclogite xenoliths have had a complex history involving Paleoproterozoicmetamorphism, thermal perturbations, and two or more episodesof Precambrian mantle metasomatism. The oldest metasomatic event(Type 1) occurred near the time of Paleoproterozoic metamorphism(1·8 Ga) and is responsible for the extreme HFSE enrichmentand growth of zircon and high-niobian rutile. A second thermalperturbation and concomitant carbonatite metasomatism (Type2) is responsible for significant apatite growth in some xenolithsand profound light rare earth element enrichment. Type 2 metasomatismoccurred in the period 1·0–1·3 Ga and isrecorded by relatively consistent whole-rock eclogite modelNd ages and secondary U–Pb zircon upper intercept dates.These eclogite xenoliths were derived from a variety of protoliths,some of which could represent metasomatized pieces of oceaniccrust, possibly linked to east-dipping subduction beneath theSlave craton during construction of the 1·88–1·84Ga Great Bear continental arc. Others, including the diamond-bearingeclogites, could be cumulates from mafic or ultramafic sillcomplexes that intruded the Slave lithospheric mantle at depthsof about 150–180 km. KEY WORDS: zircon- and diamond-bearing eclogites; Jericho kimberlite, geochronology; Precambrian metasomatism, northern Slave Craton     

REE-HFSE distribution/partitioning between garnetiferous restites and TTG from Nademavinapura area,Western Dharwar craton

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K₂O, MgO, FeO, TiO₂ and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na₂O, Al₂O₃, LREE, Th, U and enriched in K₂O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.     

Geochemistry of granitoids of Bilgi area,northern part of eastern Dharwar craton,southern India — Example of transitional TTGs derived from depleted source

Mildly deformed granitoids exposed around Bilgi in the northernmost part of the eastern Dharwar craton are divided into two groups viz. granodiorites and monzogranites. The granodiorites contain microgranular enclaves and amphibolite xenoliths, and show low-Al TTG affinity with high SiO₂ (71–74 %), Na₂O, Y and Sr/Y, moderate to moderately high Mg#, Cr and Ni, low to moderate LILE, and low Nb and Ta. However, compared to similar TTGs from different cratons the Bilgi granodiorites have distinctly higher K₂O, K₂O/Na₂O, Rb and lower REE and Th. The amphibolite xenoliths are characterized by variable enrichment of K₂O, Rb, Ba and Th and depletion of Ti, Zr and P compared to MORB. The microgranular enclaves are quartz diorite to granodiorite in composition with high Mg, Ni and Cr, and compared to MORB, are enriched in LILE and depleted in Ti and Y. The monzogranites, compared to the granodiorites, display higher SiO₂, K₂O and Rb with lower Mg#, although still maintaining the high Na₂O, Ni and Cr and low REE character. The Bilgi granodiorites are explained as transitional TTGs late synkinematic with respect to regional deformation. Geochemical signatures and regional geological set up suggest that they are probably derived from partial melting of a highly depleted slab material (metabasalt) followed by variable contamination or assimilation of intermediate crustal rocks in a subduction zone set up. Late stage fluid activity on the granodioritic magma is probably responsible for the generation of monzogranites. The amphibolite xenoliths predate the granodiorites and possibly represent fragments of a schist belt carried away by the granitic magma. They are probably island arc basalt derived from mantle source that has been metasomatized by slab-derived fluids. The microgranular enclaves are coeval with the Bilgi granodiorites and also likely to be island arc magmas derived from mantle variably enriched in slab-derived and within-plate components.     

Magnetic anomalies across Bastar craton and Pranhita-Godavari basin in south of central India

Aeromagnetic anomalies over Bastar craton and Pranhita-Godavari (P-G) basin in the south of central India could be attributed to NW-SE striking mafic intrusives in both the areas at variable depths. Such intrusions can be explained considering the collision of the Bastar and Dharwar cratons by the end of the Archaean and the development of tensile regimes that followed in the Paleoproterozoic, facilitating intrusions of mafic dykes into the continental crust. The P-G basin area, being a zone of crustal weakness along the contact of the Bastar and Dharwar cratons, also experienced extensional tectonics. The inferred remanent magnetization of these dykes dips upwards and it is such that the dykes are oriented towards the east of the magnetic north at the time of their formation compared to their present NW-SE strike. Assuming that there was no imprint of magnetization of a later date, it is concluded that the Indian plate was located in the southern hemisphere, either independently or as part of a supercontinent, for some span of time during Paleoproterozoic and was involved in complex path of movement and rotation subsequently. The paper presents a case study of the utility of aeromagnetic anomalies in qualitatively deducing the palaeopositions of the landmasses from the interpreted remanent magnetism of buried intrusive bodies.     

内蒙古敖汉旗早三叠世正长花岗岩锆石U-Pb年代学、地球化学特征

正近些年研究显示华北地台北缘存在一条近东西向展布的早三叠世后造山的花岗岩带,在吉林东部,内蒙古中部及西部地区具有发现,但在赤峰敖汉旗地区还未见报道。本次工作在敖汉旗区域地质调查过程中,从原来侏罗纪岩体中重新厘定出一套正长花岗岩,本文对其年代学及地球化学进行介绍,以探讨华北地台北缘中段-东段过渡带的古亚洲洋闭合时限。