Final-stage Southward Subduction of the Eastern Paleo-Asian Ocean: Evidence from the Middle Permian Mafic Intrusions in the Northern Margin of the North China Craton

The northern margin of the North China Craton(NCC)contains widespread Permian magmatic rocks,but the origin of these rocks remains controversial.This uncertainty hampers us from better understanding of tectonic framework and evolution of the eastern Paleo-Asian Ocean,particularly with respect to its final-stage subduction and closure time.To address these questions,this study presents petrological,zircon U-Pb geochronological,whole-rock geochemical and in situ zircon Hf isotopic data for these Permian mafic intrusions in the northern margin of the NCC.Precise zircon U-Pb dating results indicate that these mafic intrusions were emplaced in the Middle Permian(ca.260 Ma).Geochemically,the studied mafic intrusions have high MgO and transition metals element contents,with high Mg^# values,indicating a mantle origin.These mafic intrusions are characterized by enrichments in large ion lithophile elements(LILEs;e.g.,Rb,Ba,and K)and light rare earth elements(LREEs),and depletions in high field strength elements(HFSEs;e.g.,Nb,Ta,and Ti)and heavy rare earth elements(HREEs),indicating that they were formed in a subduction-related setting.These geochemical features,together with zircon ε_Hf(t)values(-1.1 to+11.2),indicate that their parental magmas were derived from partial melting of heterogeneous mantle wedge metasomatized by subduction-related fluids,with the contributions of slab sediments.The studied mafic intrusions also show wide range of major and trace elements contents,and variable Mg^# values,Eu and Sr anomalies,suggesting that their parental magmas had undergone variable degrees of fractional crystallization.Together with the E-W trending Permian continental arc along the northern margin of the NCC,we confirm that the generation of the Middle Permian mafic intrusions was related to southward subduction of the Paleo-Asian oceanic lithosphere beneath the NCC and the Paleo-Asian Ocean had not closed prior to the Middle Permian.     

甘肃省龙首山牛角沟铀矿点钾钠混合交代作用研究

牛角沟铀矿点是甘肃省龙首山铀成矿带中段东部重要的钠交代型铀矿点,该矿点外围新发现的钾钠混合交代花岗岩在该区难得一见。为更细致的解析钾钠混合交代作用,本文从岩石学、矿物学、地球化学、同位素年代学等方面开展研究,认为牛角沟中粗粒花岗岩是早古生代（445.2±7.8Ma）祁连山—龙首山造山作用晚期,后碰撞伸展环境中产出的中硅、富碱弱过铝、低铁和钙而贫钛、镁、磷的高钾钙碱性花岗岩。钾钠混合交代作用形成蚀变岩从中心向外依次为孔洞状钾钠混合交代花岗岩→雪花状方解石化钾钠混合交代花岗岩→少石英无暗色矿物紫红色钾钠混合交代花岗岩→假象绿泥石化花岗岩逐渐过渡为正常花岗岩。钾钠混合交代作用的热液溶蚀萃取了流经花岗岩中的SiO2、TiO2、FeO、Fe2O3、MnO、MgO、CaO和P2O5,以及REE、Ba、Sr、Zr、Sc、Cr、Sr、Co、Mo、Hf、Bi、Sb、Pb等化学组分,迁入孔洞状钾钠混合交代花岗岩中的组分有Na2O、K2O、MnO、Th、U、Rb、Nb、Ta、W、Sr等。CaO、MnO、Na2O、Th、U、Nb、Ta、Rb、HREE等组分与方解石一起迁入雪花状方解石化钾钠混合交代蚀变带中,而TiO2、FeO、Fe2O3、MgO、CaO、P2O5、V、U、Se、Zr等化学组分在紫红色钾钠混合交代花岗岩中析出富集。钾钠混合交代花岗岩性脆、易碎、多孔、“疏松”,为之后钠交代铀成矿作用提供了良好的围岩基础。其侵位时代进一步印证了龙首山钠交代型铀矿成矿作用起始于444~442Ma前后的可靠性。     

华北板块东南缘五河杂岩的麻粒岩相变质、部分熔融与交代作用

华北板块东南缘五河杂岩属于前寒武纪变质基底,出露于蚌埠及相邻地区,主要由变质的镁铁质和长英质火成岩以及表壳岩系组成,并伴生有古元古代片麻状花岗岩和中生代花岗岩类,构成了"蚌埠隆起"。五河杂岩中主要有石榴斜长角闪岩/榴闪岩、石榴麻粒岩、异剥钙榴岩、石榴斜长角闪片麻岩、花岗片麻岩、云母片岩、大理岩和变质砂岩等岩石类型。这些不同类型的变质岩具有类似的峰期变质矿物,如石榴子石、单斜辉石、斜长石、金红石和石英等;结合其锆石U-Pb年龄,表明它们共同经历了古元古代高压麻粒岩相变质作用。石榴斜长角闪岩的石榴子石包体中存在钾长石+斜长石+石英+黑云母等矿物交生体,指示早期花岗质熔体的存在,证明五河杂岩在古元代变质过程中发生了部分熔融作用。此外,部分变基性岩中存在方解石、绿帘石和绿泥石等蚀变矿物,沿石榴子石的裂隙中分布有钾长石+斜长石+石英+黑云母+角闪石等矿物,表明它们经历了含碳酸盐的熔/流体交代作用以及绿帘石化和绿泥石化作用等。遭受过古元古代含碳酸盐的熔/流体交代作用的变基性岩表现为高放射成因Pb同位素组成,而未遭受含碳酸盐的熔/流体交代作用的变基性岩则表现为典型华北板块前寒武纪麻粒岩地体和下地壳包体岩石的低放射成因Pb同位素组成。     

新疆望峰金矿区成矿流体的Si同位素示踪研究及其成矿意义

新疆望峰金矿区金的成矿作用与硅化作用密切相关,文章通过对矿区典型矿体的赋矿围岩、蚀变岩石及矿石进行Si同位素组成研究,探讨了成矿流体中硅质来源、演化过程及其成矿意义。结果表明:1望峰金矿体围岩具有较均一的Si同位素组成,δ30Si=-0.2‰(n=3),显示了花岗质原岩Si同位素组成特征,动力变质作用对原岩硅同位素组成无明显影响;2蚀变岩石Si同位素组成均一性较高,δ30Si=-0.1‰(n=3),显示了花岗质原岩Si与成矿期热液Si的混合特征;3矿石Si同位素组成变化范围较大,不同类型矿石的Si同位素组成差异较明显:硅化蚀变糜棱岩型δ30Si=-0.5‰~-0.2‰(n=9),集中于-0.3‰~-0.2‰,平均-0.3‰;硅化蚀变岩型δ30Si=-0.3‰~-0.1‰(n=9),平均-0.2‰;热液石英脉型δ30Si=-0.3‰~0.1‰(n=6),平均-0.1‰。望峰金矿区金成矿流体中的硅质,主要来源于花岗质赋矿围岩。矿石中Si同位素组成变化较大,是富硅成矿流体演化过程中发生Si同位素动力学分馏的结果,矿体下部及矿区深部应有一定的找矿前景。     

New evidence for two sharp replacement fronts during albitization of granitoids from northern Aravalli orogen,northwest India

We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink microcline–albite granite, and stage II converted the microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, biotite (annite-rich) and hastingsite (amphibole) to hastingsite with low X_Fe values, dehydration, gain in Na, and losses in Fe and Rb. The second stage of metasomatism caused almost complete conversion of microcline to albite and complete or nearly complete disappearance of amphibole. Chemically, these changes are manifested by substantial gain in Na and extreme losses in K, Rb, Ba, Ca, Sr, Fe, and Mg. Depending on the modal abundances of amphibole, stage II albitized rocks are depleted in light rare earth elements or heavy rare earth elements or both, signifying that rare earth elements are principally hosted by mafic phases. The disparity in whole-rock δ¹⁸O values during both stages of albitization is related to the variations in modal amounts of Si-bearing phases. The replacement microstructures are in accord with the fluid-mediated phase transformations by a coupled dissolution–precipitation mechanism. The albitizing event took place at low temperatures of 350–400 °C and the fluid was metamorphic in nature.     

Geochemistry of the Mt Wright Volcanics from the Wonominta Block,northwestern New South Wales

The Mt Wright Volcanics are located in the Wonominta Block of northwestern New South Wales. Detailed regional mapping has shown that the block is a composite tectonic unit and that the metavolcanic rocks described as the Mt Wright Volcanics may have been emplaced at different time from Late Proterozoic (northern section: Packsaddle, Nundora) to Early Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clinopyroxene, show that the rocks have affinities with alkali basalt with light‐rare‐earth‐element‐enriched compositions. An intra‐plate extensional environment (such as rift‐ and/or plume‐related) is considered most likely for the formation of the rocks. Though metamorphosed to various degrees, the rocks apparently retain much of their primary Sr isotopic character (initial ⁸⁷Sr/⁸⁶Sr about 0.7032) and, apart from their age, resemble the Tertiary intraplate volcanism in eastern Australia. The Nd isotope analyses yield remarkably similar results between the two sections of the Mt Wright Volcanics, with ¹⁴³Nd/¹⁴⁴Nd between 0.51260 to 0.51271 and _εNd(T) 4.7 ±0.4 (calculated to 525 Ma). A kaersutite‐bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with _εNd(T) of 7.7. The isotope results suggest that the Mt Wright Volcanics were derived from a depleted mantle source without significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambrian in eastern Australia.     

Metasomatism and self-organization of dolerite dyke–marble contacts: Beinn an Dubhaich, Skye

Dolerite dykes which intrude dolomitic marble in the aureole of the Beinn an Dubhaich granite on the Isle of Skye have developed metasomatized margins close to the contact with the (younger) granite in the southern part of Torrin Quarry. These margins display well-defined mineralogical zones which can be divided into two groups on the basis of composition. Chlorite-bearing margins display the sequence dolerite | diopside+spinel | chlorite+calcite+magnetite | olivine+calcite+spinel | dolomite, whereas phlogopite-bearing sequences have a phlogopite+calcite±fluorapatite layer in place of the chlorite-bearing layer. No spatial pattern is seen in the distribution of the two margin types, but textural and compositional evidence points to an importance of metasomatism by localized infiltration of magmatic fluids in the development of the phlogopite-bearing sequences. Extensive patterning of the phlogopite-bearing margins by a process of self-organization during grain growth has resulted in the formation of alternating silicate-only/calcite-only layers parallel to the dyke margin on a millimetre to centimetre scale. Early development of chlorite-bearing sequences at the dolerite–dolomite contact during contact metamorphism was followed by metasomatism-controlled growth of phlogopite and patterning at or after the peak of metamorphism associated with the expulsion of aqueous fluids from the crystallizing pluton.     

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit,Ghana

Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold‐bearing quartz veins, produced by the infiltration of a H₂O–CO₂–K₂O–H₂S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc , in subsets of the model system MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O–TiO₂–Fe₂O₃, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well‐developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO₂) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO₂). These independent P–T–X(CO₂) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO₂) of ~0.7. Fluid‐rock interaction calculations with M–X(CO₂) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.     

岩石中硼的提取分离及同位素组成的测定

介绍了一种用Ｎａ２ＣＯ３＋Ｋ２ＣＯ３混合熔剂对石样品进行分解,用硼特效树脂和阴、阳混合离子交换树脂相结合进行岩石样品中硼的纯化分离的方法。采用该方法从岩石样品中提取、分离出来的硼能满足硼同位素质谱法测定的需要,不产生硼同位素分馏,硼同位素测定精度大多优于０．０５％。     

Metamorphic processes: a celebration of the career contribution of Ron Vernon

This Special Issue is in celebration of the career contribution of Ron Vernon. It arises from the ‘Ron Vernon Symposium’, which was held as part of the 15th Australian Geological Convention in Sydney in 2000. To date Ron has published more than one hundred scientific papers, several books and contributed widely to international conferences. He is the doyen in the multidisciplinary field of microstructures, which spans petrology and structural geology. However, Ron's range is phenomenal, and he has contributed to many aspects of metamorphism, magmatism and rock deformation. He has drawn on data from his work in these areas to contribute widely to understanding thermal regimes, crustal evolution and tectonics, as exemplified by his ideas about HTLP metamorphism and emplacement of granites. Ron has followed the scientific method in his work; it has led him to many innovative interpretations, some of which undermined established dogma. By example, Ron has been a positive role model for the younger generation. However, for the older generation, he has been something of a bête noire! The papers in this Special Issue cover the range from low‐ to high‐grade metamorphism, from metasomatism to melting, from microstructures to tectonics, and from fieldwork to experiments. It is our hope that in reading them you will take time to think about what has been achieved in the past 40 years, and use this as motivation for what remains to be done.