Petrology,geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks,Luzong volcanic basin,eastern China

We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, representing a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider diagram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed.     

Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group,southern margin of the North China Craton

The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO₂ contents ranging from 52.10 wt% to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88 wt%) and accordingly low Mg^# values of 11–53. The Nd isotopes are enriched and show a weak variation with ?_Nd(t) = −7.12 to −9.63. Zircon Hf isotopes are also enriched with ?_Hf(t) = −12.02 ± 0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO₂ contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000 km²) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean.     

The origin of decoupled Hf-Nd isotope compositions in Eoarchean rocks from southern West Greenland

Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu-Hf and Sm-Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the ∼3700 Ma and ∼3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf-εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm-Nd and Lu-Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium-Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm-Nd space, indicating early disturbance of the Sm-Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf-εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf-Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf-Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf-Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites.     

长江中下游溧水盆地火山岩的时代、地球化学特征及其地质意义

溧水盆地位于长江中下游成矿带东部,与庐枞、宁芜等火山岩盆地相似,发育中生代火山岩。为了探讨溧水盆地不同旋回火山岩形成时代、岩浆源区及岩浆活动的大地构造背景及其与铁铜成矿关系等问题,本次研究选取溧水盆地广泛出露且与成矿关系密切的中生代龙王山旋回和大王山旋回火山岩进行了锆石精确定年和Hf同位素分析以及全岩主微量元素分析。研究表明溧水盆地岩浆活动起始时间为~135 Ma,峰期时间为135~124 Ma。龙王山、大王山旋回火山岩Hf同位素特征相似,εHf(t)值范围大多在-6~-11之间,暗示其源区可能相似。岩石地球化学特征显示龙王山—大王山旋回岩浆由中基性向中酸性过渡,并向高钾富碱方向演化,空间上从北西-南东方向演变,与盆地内铁铜金锶矿的形成有关。龙王山旋回火山岩明显Sr正异常,推测龙王山旋回可能是Sr矿的矿源层。结合岩石地球化学和同位素特征,认为岩浆源区为可能为富集地幔,在喷发早期受到了少量陆壳物质的混染。溧水盆地与长江中下游其它火山岩盆地形成时代相似,可能受同一构造背景控制。结合前人研究成果分析认为在~135 Ma为太平洋板块俯冲的转折点,即俯冲作用减弱,俯冲板块发生后撤,形成区域性拉张环境,从而形成的一系列火山岩盆地。     

华南晚奥陶世次火山岩的发现:早古生代与俯冲有关的英安岩?

华南早古生代花岗岩广泛分布,但同时代的(次)火山岩却很罕见.本次研究采用LA-ICPMS锆石U-Pb定年方法,结合微区原位Hf同位素分析,对南岭广东韶关地区的河口、上洞英安岩进行了系统研究.结果表明英安岩形成于448.7±1.7 Ma,为晚奥陶世岩浆活动产物.岩石为斑状结构,常见石英碎斑或港湾状石英斑晶,基质为隐晶质,具有典型浅成岩石特征.地球化学资料表明,这些岩石属于高钾钙碱性英安岩.英安岩微量元素显示了轻稀土富集、高场强元素(Ta、Nb)亏损和极低的Cr、Ni含量等明显火山弧岩浆特征.同位素数据显示河口、上洞英安岩全岩I_Sr=0.709 0~0.714 0,ε_Nd(t)=-8.9~-9.3,锆石ε_Hf(t)变化范围为-8.06~-3.80,与华南加里东期主要花岗岩类同位素特征一致,具有壳源属性.二阶段Hf模式年龄范围为1.67~1.93 Ga,与Nd模式年龄结果1.91~1.94 Ga相近;表明河口、上洞英安岩来源于古元古代结晶基底地壳物质.河口、上洞奥陶纪英安岩的发现,进一步证明该区早古生代存在俯冲岩浆活动,为研究华南早古生代地质事件提供了新的证据.      

西藏冈底斯西段狮多地区火山岩的地质地球化学特征

西藏冈底斯西段狮多地区火山岩为一套高钾钙碱性和钙碱性系列岩石,分为古新世(59 7Ma)和中新世(17 2Ma)两个活动时期,早期的时代和层位大致相当于冈底斯东段的林子宗群典中组。与典中组火山岩相比较,该期火山岩的稀土元素总量(w(∑REE)=58 83×10-6)低得多,轻、重稀土分馏程度(w(La)N/w(Yb)N=3 44)较弱,分布曲线更为平缓;N(143Nd)/N(144Nd)的现在值(0 512275～0 512420)和初始值εNd(t)(-6 7～-4 0)也明显偏低,这反映冈底斯东、西段同一时期的火山活动及演化各具特色。根据Pb、Nd、Sr同位素的组成特征和微量元素构造环境判别推断,狮多地区火山岩形成于陆缘弧—陆 陆碰撞造山的发展演化环境,是新特提斯洋壳向大陆下俯冲作用过渡到陆 陆碰撞造山作用的产物。狮多铁 铜矿与古新世火山岩有成因联系。     

羌塘东部治多县直根尕卡一带二叠纪栖霞期火山岩地球化学特征及其构造意义

羌塘东部治多县直根尕卡一带二叠纪栖霞期火山岩主要由中基性火山碎屑岩及熔岩组成,火山岩地球化学研究表明,其主元素表现为高TiO_2和低TiO_2两种特征,球粒陨石标准化稀土配分模式为LREE富集型.MORB标准化的微量元素配分型式为大洋隆起型,显示岩浆形成于板内裂谷构造环境.Sr、Nd、Pb同位素地球化学研究表明火山岩显示明显的亏损地幔源区特征.综合研究表明直根尕卡一带二叠纪栖霞期火山岩形成于大陆边缘拉张构造(或陆缘始裂谷)环境,岩浆起源于地幔,属地幔柱作用的产物.     

Contrasting provenance of Late Archean metasedimentary rocks from the Wutai Complex, North China Craton: detrital zircon U–Pb, whole-rock Sm–Nd isotopic, and geochemical data

Detrital zircon U–Pb ages, whole-rock Nd isotopic, and geochemical data of metasedimentary rocks from the Wutai Complex in the Central Zone, North China Craton, have been determined. Compositionally, these rocks are characterized by a narrow variation in SiO₂/Al₂O₃ (2.78–3.96, except sample 2007-1), variable Eu anomalies, spanning a range from significantly negative Eu anomalies to slightly positive anomalies (Eu/Eu* = 0.58–1.12), and positive ε _Nd (t) values (0.1–1.97). The 18 detrital zircons of one sample yielded age populations of 2.53 Ga, 2.60 Ga, and 2.70–2.85 Ga. Geochemical data reveal intermediate source weathering, varying degrees of K-metasomatism in the majority of these metasedimentary rocks, whereas other secondary disturbances seem to be negligible. Detailed analysis in detrital zircon U–Pb geochronology, whole-rock Nd isotope, and geochemistry shows that these metasedimentary rocks are derived from a mixed provenance. The predominant derivation is from the late Archean granitoids and metamorphic volcanics in the Wutai Complex, and there is also input of older continental remnants, except TTG gneisses, from the Hengshan and Fuping Complexes. The sediments were probably deposited in fore-arc or/and intra-arc basins within an arc system.     

青藏高原岩石圈演化的记录：藏北超钾质及钠质火山岩的岩石学与地球化学特征

藏北鱼鳞山超钾质火山岩为一中等规模的熔岩被, 可分为三期。前两期主要为霞石黝方白榴石响岩、霞石白榴斑岩等, 后期主要为方钠石霓石粗面岩、霓霞粗面岩等。时代为１８～３０ Ｍａ。本区超钾质岩浆 Ｓｉ Ｏ２ 强烈不饱和、强碱性、高度富集 Ｌ Ｒ Ｅ Ｅ及 Ｌ Ｉ Ｌ Ｅ、放射成因 Ｓｒ、 Ｐｂ 及非放射成因 Ｎｄ 同位素 （８６ Ｓｒ／８７ Ｓｒ＝ ０７０８７６６～０７０９１６２, ２０６ Ｐｂ／２０４ Ｐｂ＝ １８８８２８８８～ １９１０４７５１, ２０７ Ｐｂ／２０４ Ｐｂ＝ １５６４２０３７ ～ １５８２８４５８, ２０８ Ｐｂ／２０４ Ｐｂ＝ ３９１３８９２１ ～ ３９６８６８３５ 及１４３ Ｎｄ／１４４ Ｎｄ＝ ０５１２１１６ ～０５１２３４２）,其上述特征指示岩浆源区为受到地壳物质强烈交代的富集 Ｅ Ｍ Ｉ Ｉ型地幔。藏北拉嘎拉钠质碱性橄榄玄武岩为高原面上的剥蚀残丘, 时代为６０ Ｍａ。其岩石学、 Ｒ Ｅ Ｅ、微量元素及 Ｓｒ Ｎｄ Ｐｂ 同位素指示了岩浆源区为受到洋壳俯冲物质交代的轻度富集地幔。本研究结果指示, 羌塘高原南部富集地幔的形成可能是受到印度板块俯冲的影响。     

Proterozoic volcanism in the Jack Hills Belt,Western Australia: Some implications and consequences for the World's oldest zircon population

Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9 Ga. The ∼3.3–3.4 Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3 Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1 Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6 Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775 Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0 Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4 Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area.     

豫西地区秦岭造山带武当群Nd-Hf同位素组成及其物源特征

武当群变质沉积-火山岩组合是南秦岭地体中重要的基底岩石,其形成时代和地球化学特征可以为理解秦岭造山带的构造演化提供重要的证据.本文报道豫西地区武当群上部沉积岩和下部中-酸性火山岩Sm-Nd同位素和锆石Lu-Hf同位素组成,探讨火山岩成因和沉积岩物源的同位素特征.上部沉积岩的碎屑锆石初始ε_(Hf)值变化在-30～+10之间,对应的模式年龄值t_(DM2)在1.0Ga至3.2Ga之间,初始ε_(Nd)值在-4.0至-6.0之间.沉积物源表现为主要与扬子陆块有亲缘关系的地壳物质和近源的下部火山岩混合的特征.火山岩的锆石初始εHf值变化在-35～+15之间,对应的模式年龄值t_(DM2)在0.8Ga至3.5Ga之间,集中于1.5～1.8Ga和2.2～2.4Ga两个峰值.2个变质石英角斑岩样品初始ε_(Nd)值分别为-9.2和-10.7,而报道的湖北武当群的玄武-安山质熔岩的初始ε_(Nd)值以正值为主.因此,武当群不同类型的火山岩可能存在着成因差异.具有低初始ε_(Nd)值和ε_(Hf)值特征的火山岩可能由地壳物质的重熔而形成的;有些火山岩具有初始ε_(Hf)值变化范围较大(-35～+15)或正初始ε_(Nd)值的特点,可能是壳、幔物质混合成因,有显著的幔源或新生地壳物质的贡献.武当群Nd-Hf同位素组成和碎屑锆石年龄分布特征表明,与扬子陆块有亲缘关系的南秦岭地体在元古代期间可能经历多期地壳增生和再造作用.     

The Hf-Nd isotopic composition of marine sediments

This paper presents new major and trace-element data and Lu-Hf and Sm-Nd isotopic compositions for representative suites of marine sediment samples from 14 drill sites outboard of the world’s major subduction zones. These suites and samples were chosen to represent the global range in lithology, Lu/Hf ratios, and sediment flux in subducting sediments worldwide. The data reported here represent the most comprehensive data set on subducting sediments and define the Hf-Nd isotopic variations that occur in oceanic sediments and constrain the processes that caused them.Using new marine sediment data presented here, in conjunction with published data, we derive a new Terrestrial Array given by the equation, ε_Hf = 1.55 × ε_Nd + 1.21. This array was calculated using >3400 present-day Hf and Nd isotope values. The steeper slope and smaller y-intercept of this array, compared to the original expression (ε_Hf = 1.36 × ε_Nd + 2.89; Vervoort et al., 1999) reflects the use of present day values and the unradiogenic Hf of old continental samples included in the array.In order to examine the Hf-Nd isotopic variations in marine sediments, we have classified our samples into 5 groups based on lithology and major and trace-element geochemical compositions: turbidites, terrigenous clays, and volcaniclastic, hydrothermal and hydrogenetic sediments. Compositions along the Terrestrial Array are largely controlled by terrigenous material derived from the continents and delivered to the ocean basins via turbidites, volcaniclastic sediments, and volcanic inputs from magmatic arcs. Compositions below the Terrestrial Array derive from unradiogenic Hf in zircon-rich turbidites. The anomalous compositions above the Terrestrial Array largely reflect the decoupled behavior of Hf and Nd during continental weathering and delivery to the ocean. Both terrigenous and hydrogenetic clays possess anomalously radiogenic Hf, reflecting terrestrial sedimentary and weathering processes on the one hand and marine inheritance on the other. This probably occurs during complementary processes involving preferential retention of unradiogenic Hf on the continents in the form of zircon and release of radiogenic Hf from the breakdown of easily weathered, high Lu-Hf phases such as apatite.     

Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)

Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects.⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, ranging 0.705-0.709 and 0.5124-0.5130 for Blue Nile tributaries, are 0.704-0.705 and 0.5127-0.5128 for largely volcaniclastic sediments of River Tekeze-Atbara, and 0.705-0.706 and 0.5126-0.5127 for main Nile sediments upstream Lake Nasser.Model mantle derivation ages (t_DM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4-3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4-2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6-1.3 Ga (White Nile upstream Khartoum). Instead, t_DM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. T_DM ages are younger for sediments shed by Ethiopian (1.2-0.7 Ga) and Eritrean basements (1.5-1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3-0.2 Ga).Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and t_DM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand.The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.     

Chlorine in submarine volcanic glasses from the eastern manus basin

Submarine volcanic glasses from the eastern Manus Basin of Papua New Guinea, ranging from basalt to rhyodacite, clarify the geochemical behavior of Cl in arc-type magmas. For the Manus samples, Cl is well correlated with non-volatile highly incompatible trace elements, suggesting it was not highly volatile and discounting significant seawater contamination. The Cl partition coefficient is close to but slightly lower than that of Nb and K₂O, a behavior similar to that in mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). The similar incompatibilities of Cl and Nb imply that the Cl/Nb values of the eastern Manus Basin glasses reflect their magma source. For glasses from other west Pacific back-arc basins, Cl/Nb, Ba/Nb, and U/Nb increase towards the subduction trench, indicating increased contribution of a component enriched in Cl, Ba, and U, likely from subduction-released slab fluids. It is estimate that ∼80% of the Cl in the Manus arc-type glasses was added directly from subducted slab-derived fluids. We have also modeled Cl behavior during magma evolution in general. Our results show that the behavior of Cl in magma is strongly influenced by pressure, initial H₂O content, and the degree of magmatic fractionation. At early stages of magmatic evolution, for magmas with initial H₂O content of <4.0 wt%, Cl is highly incompatible under all pressures. By contrast, for more evolved magmas at moderately high pressure and high H₂O contents, considerable amounts of Cl can be extracted from the magma once H₂O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB because of their low H₂O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H₂O content and depth of magma chambers. The behavior of Cl in the eastern Manus Basin magmas is consistent with low H₂O contents (1.1-1.7 wt%) and evolution at low pressures (<0.1 GPa). Modeling results also indicate that Cl will behave differently in intrusive rocks compared to volcanic rocks because of the different pressures involved. This may have a strong influence on the mechanisms of ore genesis in these two tectonic settings.     

华南花岗岩-火山岩成因研究的几个问题

华南花岗岩-火山岩具幕式多期次产出的特点,成因类型和形成过程错综复杂。与世界其它地区相比,华南花岗岩-火山岩演化及其成矿作用是有鲜明特色的。近半个世纪的研究,逐步认识了华南花岗岩-火山岩的多样性及其时空分布格架,初步认识到其多样性受控于不同的地壳基底物质成分、不同的构造动力学背景、不同的壳幔相互作用过程等因素。但由于华南地质的特殊性和复杂性,不少具体的成岩成矿物质来源、空间和构造背景,甚至基础地质理论问题有待解析。华南深部地质及围绕华南的构造边界条件对花岗岩-火山岩成因的制约是目前的研究薄弱环节。本文提出了与中-新生代地质有关的值得研究的几个问题,包括：（1）三条低Nd模式年龄带及A型花岗岩;（2）四条火山岩带及壳幔相互作用;（3）全岩Nd同位素和锆石Hf同位素示踪;（4）花岗岩构造地质。     

The 2.73 Ga I-type granites in the Lengshui Complex and implications for the Neoarchean tectonic evolution of the Yangtze Craton

ABSTRACT

We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 ± 13 Ma and 2738 ± 25 Ma, respectively. The more precise age of 2732 ± 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO₂ (73.11–74.01 wt%), K₂O (3.93–5.48 wt%), Na₂O (3.93–4.86 wt%) and low CaO (0.30–0.69 wt%), MgO (0.17–0.30 wt%), TiO₂ (0.14–0.17 wt%), P₂O₅ (0.01–0.06 wt%), Al₂O₃ (14.11–14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04–1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2–34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their ε_Hf(t) values (?2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive ε_Nd(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67–2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.     

Petrology and geochemistry of the Mesoproterozoic Vattikod lamproites,Eastern Dharwar Craton,southern India: evidence for multiple enrichment of sub-continental lithospheric mantle and links with amalgamation and break-up of the Columbia supercontinent

Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW–ESE to NW–SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K₂O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial ⁸⁷Sr/⁸⁶Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ?Nd range from ??10.6 to ??9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic–anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history.     

Lu-Hf apatite geochronology of mafic cumulates: An example from a Fe-Ti mineralization at Smålands Taberg, southern Sweden

We have employed Lu-Hf apatite chronology to determine the age of a magnetite-rich melatroctolite in southern Sweden, known as the Smålands Taberg Fe-Ti ore deposit. The melatroctolite is poor in incompatible elements and minerals forming at more advanced stages of differentiation, which precludes the use of minerals (baddeleyite, zircon, titanite or apatite) commonly used for dating the emplacement of igneous rocks. Enclaves of leucogabbro in the melatroctolite have REE patterns parallel with the host melatroctolite and identical initial Hf and Nd isotope compositions. These characteristics are conclusive evidence for a common parental magma. The leucogabbro is a slightly more evolved facies that underwent limited fractionation of olivine and titanomagnetite; the two major mineral phases in the melatroctolite. Apatite and plagioclase, separated from the leucogabbro, plus a whole-rock sample define a Lu-Hf isochron with a slope corresponding to an age of 1204.3 ± 1.8 Ma. This result is close to the lower age boundary previously reported for this magmatic event comprising dolerites, syenites and granites. The Lu-Hf apatite chronometer merits attention as a new tool in generating accurate and precise ages for igneous, silica-undersaturated rocks that may be difficult to date with traditional techniques.     

Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time

The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation ε_Hf = 1.40 ε_Nd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of ε_Hf = 1.36 ε_Nd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in ε_Nd values (⁻4.4 to ⁺4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The ε_Hf values for these rocks are consistently positive and have much less variation (0 to ⁺3.4) than their ε_Nd counterparts.The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks.Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive ε_Hf and ε_Nd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.     

Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia.

Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate.

Inherent in the above chemical geodynamic models are the high ?_Nd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.