The lithospheric mantle beneath the southwestern Tianshan area, northwest China

A suite of spinel peridotite xenoliths in Mesozoic basalts of the Tuoyun basin in the Tianshan area of northwest China has a high proportion of amphibole/mica-bearing lherzolites, with high Cpx/Opx ratios (mean 0.74). Many aspects of mineral chemistry in the Tuoyun peridotites are intermediate between those of refractory Archean cratonic mantle and fertile Phanerozoic mantle. These include Ni/Cr and the contents of transition metals and Y in olivine and orthopyroxene and the abundances of elements such as Na, Al, Ti, Y, Sr and LREE in clinopyroxene. The data suggest that the mantle in Tuoyun is moderately depleted in basaltic components relative to both the refractory Archean mantle and the fertile Phanerozoic mantle. The wide variations in the CaO/Al₂O₃ (0.9–3.5) of whole rocks and LREE/HREE (0.8–14.2) and Ti/Eu (971–5,765) of clinopyroxenes in the Tuoyun peridotites are interpreted as the metasomatism of hydrous carbonatitic and potassic melt or the cumulative effects of mantle metasomatism by different agents (carbonatite and small-volume silicate melts) through time. The Tuoyun mantle shows closer affinity to the type of mantle found beneath the Proterozoic Cathaysia block, and especially to that beneath the East Central Asia Orogenic Belt (ECAOB), than to the mantle beneath the Archean North China Craton. This implies that the Tianshan subcontinental lithospheric mantle may have been generated during the accretion of the ECAOB. The high proportion of fine-grained microstructures, high Cpx/Opx ratio, obvious Ca enrichment and lower overall depletion in the Tuoyun mantle relative to that in other parts of the ECAOB reflect stronger mechanical and chemical modification of the Tuoyun mantle, near the translithospheric Talas-Ferghana strike-slip fault, which played a major role in controlling the strength of the mantle lithosphere and has channeled the upwelling mantle.     

华北克拉通中生代破坏前的岩石圈地幔与下地壳

华北克拉通是世界上最古老的克拉通之一,有 38亿年的古老陆壳存在,它经历了复杂的地质变迁,在太古宙末(约2500Ma)基本完成克拉通化,在古元古代(约1900~1850Ma)整体受到了高级变质作用,最终完成了克拉通化。它的东部在中生代发生了重大的构造机制的转变,克拉通基底发生了破坏、置换和再造。在太行山重力梯度带以西的华北克拉通受中生代构造转折的改造程度较低,它们的下地壳和岩石圈地幔结构,大致保持了华北克拉通破坏前的状态。前寒武纪麻粒岩地体代表了掀翻抬升到地表的古元古代下地壳,出露地表的时间大致在1850~1800Ma。中、新生代火山岩中的地幔和麻粒岩捕虏体代表了现代的岩石圈地幔和下地壳的岩石。岩石学、地球化学和地球物理的研究,推测华北克拉通西部的岩石圈厚约200km,地壳厚度约45km~50km,是在古元古代(约1.9Ga)时期终极克拉通化作用形成的,其厚度和结构与全球典型的元古宙克拉通岩石圈相同。而太行山重力梯度带以东的克拉通岩石圈地幔受到程度不等的交代、改造、置换和减薄,下地壳大规模重熔,地壳厚度也发生减薄,指示了强烈的壳幔解耦、物质交换和重新耦合的过程。     

The evolution of lithospheric mantle beneath the Kalahari Craton and its margins

The compositional structure and thermal state of the subcontinental lithospheric mantle (SCLM) beneath the Kalahari Craton and the surrounding mobile belts have been mapped in space and time using >3400 garnet xenocrysts from >50 kimberlites intruded over the period 520–80 Ma. The trace-element patterns of many garnets reflect the metasomatic refertilisation of originally highly depleted harzburgites and lherzolites, and much of the lateral and vertical heterogeneity observed in the SCLM within the craton is the product of such metasomatism. The most depleted, and possibly least modified, SCLM was sampled beneath the Limpopo Belt by early Paleozoic kimberlites; the SCLM beneath other parts of the craton may represent similar material modified by metasomatism during Phanerozoic time. In the SW part of the craton, the SCLM sampled by “Group 2” kimberlites (>110 Ma) is thicker, cooler and less metasomatised than that sampled by “Group 1” kimberlites (mostly ≤95 Ma) in the same area. Therefore, the extensively studied xenolith suite from the Group 1 kimberlites probably is not representative of primary Archean SCLM compositions. The relatively fertile SCLM beneath the mobile belts surrounding the craton is interpreted as largely Archean SCLM, metasomatised and mixed with younger material during Paleoproterozoic to Mesoproterozoic rifting and compression. This implies that at least some of the observed secular evolution in SCLM composition worldwide may reflect the reworking of Archean SCLM. There are strong correlations between mantle composition and the lateral variations in seismic velocity shown by detailed tomographic studies. Areas of relatively low Vp within the craton largely reflect the progressive refertilisation of the Archean root during episodes of intraplate magmatism, including the Bushveld (2 Ga) and Karroo (ca. 180 Ma) events; areas of high Vp map out the distribution of relatively less metasomatised Archean SCLM. The relatively low Vp of the SCLM beneath the mobile belts around the craton is consistent with its fertile composition. The seismic data may be used to map the lateral extent of different types of SCLM, taking into account the small lateral variations in the geotherm identified using the techniques described here.     

Source of metals in the Guocheng gold deposit,Jiaodong Peninsula,North China Craton: Link to early Cretaceous mafic magmatism originating from Paleoproterozoic metasomatized lithospheric mantle

Widespread Mesozoic Au and other hydrothermal polymetal (Zn–Pb–Cu–Mo–Ag–W–Fe–REE) deposits or smaller prospects occur in association with ancient mobile belts surrounding and cutting through the North China Carton (NCC). Among these, the gold ores of the Jiaodong Peninsula, Shandong Province, eastern NCC, represent the largest gold district in China. However, the genesis of these important gold mineralizations has remained controversial, notably their relationships to widespread mafic magmatism of alkaline affinity.The ore bodies of the Guocheng gold deposit on the Jiaodong Peninsula are fracture-controlled, sulfide-rich veins and disseminations, formed contemporaneously with abundant dolerite, lamprophyre and monzonite dikes at ca. 120 Ma. Dolerite dikes possess mantle-like major element compositions and alkaline affinity, associated with prominent subduction-type trace element enrichments. The dikes show petrographic and chemical evidence of magma mixing that triggered exsolution of magmatic sulfide and anhydrite crystallization, preserved as primary inclusions in phenocrysts. LA-ICP-MS analysis of magmatic sulfide inclusions demonstrates that metal abundance ratios (Ag, As, Au, Bi, Co, Cu, Mo, Ni, Pb, Sb, Zn) largely correspond to those of both unaltered bulk rock and bulk ore. Together with identical Pb isotope ratios of dolerite and bulk ore, this demonstrates that gold mineralization and dolerite dikes share a common source.Lead isotope signatures of the ore sulfides are much less radiogenic (17.08 < ²⁰⁶Pb/²⁰⁴Pb < 17.25, 15.41 <²⁰⁷Pb/²⁰⁴Pb < 15.45, 37.55 < ²⁰⁸Pb/²⁰⁴Pb < 37.93) relative to the Pb signature of Phanerozoic convecting mantle and plot to the left of the Geochron and above the MORB-source mantle Pb evolution line. Forward Monte Carlo simulations indicate three events for the U–Th–Pb isotope evolution: (1) late Archean formation of juvenile crust is followed by (2) subduction of this aged crust at ca. 1.85 Ga along with the assembly of Jiao–Liao–Ji mobile belt (suture within Columbia supercontinent). This late-Archean subducted crust released fluids with drastically reduced U/Pb that metasomatized the overlying depleted mantle, which formed cratonic lithospheric mantle. This metasomatized lithospheric mantle was (3) tapped in response to early Cretaceous extensional tectonics affecting notably the eastern margin of the NCC to generate mafic magmas and associated gold mineralization at Guocheng. Similarly non-radiogenic uranogenic Pb isotope data characterize the contemporaneous mafic dikes and gold deposits in the entire Jiaodong Peninsula, suggesting that our genetic model applies to the entire Jiaodong gold district.We propose that early Cretaceous melting of subcontinental lithospheric mantle metasomatized by subduction fluids during Paleoproterozoic amalgamation of terranes to the eastern NCC along with Columbia supercontinent assembly generated mafic magmatism and associated gold deposits. Given the conspicuous association of Phanerozoic hydrothermal ore deposits associated with reactivated Paleoproterozoic mobile belts, we envisage that our genetic model, which largely corresponds to that which is proposed for the Bingham porphyry-Cu–Au–Mo deposit, USA, may explain much of the magmatic-hydrothermal activity and associated ore formation all around the NCC.     

Early Cretaceous gabbroic rocks from the Taihang Mountains: Implications for a paleosubduction-related lithospheric mantle beneath the central North China Craton

SHRIMP zircon U–Pb ages and geochemical and Sr–Nd–Pb isotopic data are presented for the gabbroic intrusive from the southern Taihang Mountains to characterize the nature of the Mesozoic lithospheric mantle beneath the central North China Craton (NCC). The gabbroic rocks emplaced at 125 Ma and are composed of plagioclase (40–50%), amphibole (20–30%), clinopyroxene (10–15%), olivine (5–10%) and biotite (5–7%). Olivines have high MgO (Fo = 78–85) and NiO content. Clinopyroxenes are high in MgO and CaO with the dominant ones having the formula of En_42–46Wo_41–50Fs_8–13. Plagioclases are dominantly andesine–labradorite (An = 46–78%) and have normal zonation from bytownite in the core to andesine in the rim. Amphiboles are mainly magnesio and actinolitic hornblende, distinct from those in the Precambrian high-pressure granulites of the NCC. These gabbroic rocks are characterized by high MgO (9.0–11.04%) and SiO₂ (52.66–55.52%), and low Al₂O₃, FeOt and TiO₂, and could be classified as high-mg basaltic andesites. They are enriched in LILEs and LREEs, depleted in HFSEs and HREEs, and exhibit (⁸⁷Sr/⁸⁶Sr)_i = 0.70492–0.70539, ε_Nd(t) = − 12.47–15.07, (²⁰⁶Pb/²⁰⁴Pb)_i = 16.63–17.10, Δ8/4 = 70.1–107.2 and Δ7/4 = − 2.1 to − 9.4, i.e., an EMI-like isotopic signatures. Such geochemical features indicate that these early Cretaceous gabbroic rocks were originated from a refractory pyroxenitic veined-plus-peridotite source previously modified by an SiO₂-rich melt that may have been derived from Paleoproterozoic subducted crustal materials. Late Mesozoic lithospheric extension might have induced the melting of the metasomatised lithospheric mantle in response to the upwelling of the asthenosphere to generate these gabbroic rocks in the southern Taihang Mountains.     

华北中、新生代岩石圈地幔的交代作用：含金云母地幔岩提供的证据

本文对华北克拉通三个不同地区（河北汉诺坝、内蒙古集宁三义堂、河南鹤壁）新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔.     

华北克拉通岩石圈地幔的锂同位素特征与熔体改造作用

华北克拉通是显生宙以来全球古老克拉通破坏最为剧烈的地区。华北克拉通破坏及其相关的科学问题引起了国内外地质学家的广泛关注。有关华北克拉通破坏的研究已取得了许多重要进展,使我们认识到其破坏不仅表现为岩石圈厚度的剧烈减薄,更重要的是岩石圈地幔的物质组成与性质发生了巨大转变,即从古生代克拉通型转变为新生代大洋型。本文在综述华北地幔捕虏体锂同位素地球化学特征的基础上,进一步揭示了华北岩石圈地幔高度不均一的组成特征,以及不同来源的熔体对岩石圈地幔的改造作用,为深入认识华北岩石圈地幔的转变过程提供进一步的制约。     

Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province,Russia)

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO₂, Cr₂O₃, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.     

Conditions of sulfide formation in the metasomatized mantle beneath East Antarctica

Garnet–spinel lherzolites from Antarctica and peridotites from Mongolia were fluid saturated, which is indicated by the presence of fluid inclusions in their minerals. Flows of reactive fluids caused extensive metasomatic alteration of mantle materials. The cryometric and Raman spectroscopic investigation of the Antarctic xenoliths showed that their fluid was a complex mixture of CO₂, N₂, H₂S, and H₂O with a density of up to 1.23 g/cm³. The entrapment of fluids was accompanied by the formation of clusters of numerous sulfide inclusions. The compositions of these inclusions correspond to a Ni-rich sulfide melt and a monosulfide solid solution. The partition coefficient of Ni between them (D_Ni mss/melt) ranges from 0.99 to 3.23, which suggests that the two-phase sulfide assemblages in the partly decrepitated inclusions equilibrated at 920–1060°C. In order to refine the initial P-T conditions of the development of the Antarctic peridotites, the results of our investigation were evaluated in the light of experimental data on (1) the stability field of the two-phase assemblage mss + sulfide melt, (2) the solidus of peridotite + 0.9CO₂ + 0.1 H₂O, and (3) isochores of 0.8CO₂ + 0.2N₂ fluid. The obtained parameters are close to 1270–1280°C and 2.2 GPa and lie near the Sp–Gar boundary. The temperature of the existence of sulfide melt at a pressure of 2.2 GPa must be near 1300°C and corresponds to the boundary between the occurrence of carbon as CO₂ fluid and carbonate (carbonate melt).     

Highly heterogeneous lithospheric mantle beneath the Central Zone of the North China Craton evolved from Archean mantle through diverse melt refertilization

High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7031–0.7048, ¹⁴³Nd/¹⁴⁴Nd = 0.5130–0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re–Os isotopic data for Cenozoic basalt-borne xenoliths show T_RD ages of 3.0–1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2–0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6–0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re–Os data of sulfides in Hannuoba peridotites suggest that whole-rock Re–Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr–Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr–Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.     

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m³ for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. ¹⁴³Nd/¹⁴⁴Nd_i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). ⁸⁷Sr/⁸⁶Sr_i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb_i isotope ratios of the enriched samples are rather radiogenic (²⁰⁶Pb/²⁰⁴Pb_i 18.8–20.6, ²⁰⁷Pb/²⁰⁴Pb_i 15.6–15.7, ²⁰⁸Pb/²⁰⁴Pb_i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of ²⁰⁸Pb/²⁰⁴Pb_i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.     

华北东南部中生代岩石圈地幔性质、组成、富集过程及其形成机理

本文通过对我国华北东南部中生代幔源岩浆活动的时空分布规律及其地球化学特征的系统总结来进一步厘定该地区中生代岩石圈地幔的性质和组成,并通过与华北内部如鲁中地区中生代岩石圈地幔的对比研究探讨华北东部岩石圈的时空演化规律、富集过程及其形成机理。幔源岩石的 Sr-Nd-Ph 同位素特征表明华北东部中生代岩石圈地幔存在明显的时空不均匀性,其中心部位如鲁中地区以弱富集地幔为主体;而东南部如鲁西南和胶东地区则为类似 EM2型地幔(~(87)Sr/~(86)Sr_i 可高达0.7114)。华北东南部中生代岩石圈地幔随时间的演化特征也很明显。这些幔源岩石的地球化学特征和玄武岩中地幔岩捕虏体(橄榄岩和辉石岩)和捕虏晶(橄榄石和辉石)的组成和结构特征皆证明华北东南部中生代岩石圈地幔曾受到过富硅熔体的强烈改造。橄榄岩-熔体的相互反应是该区岩石圈改造和组成转变的重要方式,从而造成古生代高镁橄榄岩转变为晚中生代低镁橄榄岩和辉石岩。进入岩石圈地幔的熔体具下/中地壳物质重熔的特征,从而导致该区晚中生代岩石圈地幔的快速富集。有关华北东部中生代岩石圈减薄和改造的时限、过程和机制等问题也进行较详细的讨论。     

华北克拉通西部陆块北缘新生代岩石圈地幔特征的初步研究

凉城、四子王旗、三义堂和大同的地幔包体的岩石学和矿物的主、微量元素成分显示华北克拉通中、西部北缘总体为过渡型岩石圈地幔,为原始地幔经过不同程度的熔体抽取和后期交代富集作用的残留.凉城岩石圈地幔经历的熔体抽取程度最低,后期交代富集作用比较强烈,这与其橄榄岩中尖晶石的Cr#较低,并且共存的单斜辉石的Mg#较低一致.三义堂岩石圈地幔经历的熔体抽取程度最高,后期交代富集作用最强烈,不同样品甚至同一样品中不同单斜辉石颗粒的La/Yb和LREE配分模式变化范围非常大,且矿物边部比核部更富集微量元素.大同、四子王旗岩石圈地幔经历的熔体抽取程度介于凉城和三义堂之间,但后期的交代富集作用明显不如凉城和三义堂强烈.研究区的岩石圈地幔主要受硅酸盐熔体交代作用的影响,只有三义堂岩石圈地幔还可能也受到了碳酸盐交代作用的影响.     

西克尔橄榄岩包体揭示塔里木西北缘岩石圈地幔经历多期次交代改造

西克尔新生代玄武岩中的二辉橄榄岩包体矿物主量元素、单斜辉石（Cpx）微量元素及Sr同位素被用来揭示塔里木西北缘岩石圈地幔经历的交代改造作用。依据岩相学和矿物化学特征,西克尔二辉橄榄岩可分为A、B、C三个组。A组二辉橄榄岩具有较高的橄榄石含量（57%~65%）和Mg^#（>90）,代表了较为古老且适度难熔的岩石圈地幔,其Cpx富集大离子亲石元素,具有较高的（La/Yb）_N、低的Ti/Eu比值,指示碳酸盐交代。根据Cpx的Sr同位素及高场强元素（HFSE）的异常特征,A组又可以细分为两个亚组（A1和A2）：A1亚组Cpx核部呈现高的⁸⁷Sr/⁸⁶Sr（0.70451~0.70485）和HFSE的强烈负异常,而A2亚组Cpx核部呈现低的⁸⁷Sr/⁸⁶Sr（0.70353~0.70389）和HFSE的轻微负异常,分别代表着俯冲沉积碳酸盐和软流圈来源碳酸盐交代作用。B组和C组二辉橄榄岩具有较低的橄榄石Mg^#（< 90）,代表了经受改造后饱满的岩石圈地幔。其中B组的Cpx具极低的Zr/Hf,以及与A1亚组相似的特征,可能与蚀变洋壳熔融所产生富CO₂硅酸盐熔体交代有关;C组尖晶石高的TiO₂含量和Cpx高的重稀土含量,指示明显高熔/岩比的熔体-橄榄岩反应印迹,Cpx勺型的轻稀土配分、较低的（La/Yb）_N、高的Ti/Eu比值、及HFSE强烈亏损的特征,表明其后期经历小体积富水硅酸盐熔体渗透的影响。此外A组Cpx的海绵边和B、C组包体与寄主岩交界处Cpx的反应边呈现与核部不一致的成分特征,同时B、C组包体与寄主岩交界处的斜方辉石也呈现被替代的现象。这些指示了橄榄岩在被携带至地表前不久或在此过程中熔体渗透引发的Cpx不一致熔融及熔岩反应的改造过程。多期次的地幔交代作用指示塔里木西北缘岩石圈地幔所经历的复杂改造历史。

     

华北克拉通北缘早白垩世金成矿与地幔物质的贡献

华北克拉通北缘是我国重要的金成矿带,早白垩世金矿床主要分布在成矿带的中-东段赤峰-朝阳矿集区、五龙-猫岭矿集区、夹皮沟矿集区、老岭矿集区及附近区域。本文综述了华北北缘早白垩世金矿床的地质及同位素地球化学特征,讨论了早白垩世地幔组分参与金成矿的证据和过程。研究表明,华北北缘早白垩世金矿床与矿区内中-基性岩脉具有密切的时空关系;金矿脉中石英流体包裹体氢-氧同位素、碳酸盐矿物碳同位素及含金黄铁矿氦-氩同位素组成揭示成矿流体以岩浆水为主,具有壳-幔混合来源的特征;硫-铅-锶-钕同位素组成显示成矿物质具有显著的上地幔贡献。个别金矿床原位硫同位素组成反映成矿流体经历了强烈的水-岩反应,从围岩中萃取了部分硫等成矿组分。总体上,华北北缘早白垩世金矿具有基本一致的地质和地球化学特征,反映了幔源岩浆对金矿床的成矿流体及成矿物质具有重要贡献,其金成矿过程与早白垩世（140~120Ma）华北克拉通破坏及相关的壳幔相互作用有关。

     

Oxidized Late Mesozoic subcontinental lithospheric mantle beneath the eastern North China Craton: A clue to understanding cratonic destruction

Despite the critical influence of oxidation state on the geochemical and geodynamic evolution of Earth, its impact on the longevity of cratons is poorly understood. To address this issue, we investigated the redox state of the Late Mesozoic subcontinental lithospheric mantle (SCLM) beneath the eastern North China Craton (NCC), which was destroyed during the Phanerozoic. We report the occurrence of high-Fo olivine (Fo > 87, where Fo = atomic Mg/(Mg + Fe²⁺)) within Early Cretaceous basalts; these olivines show extremely low Ti (<60 ppm) contents, high δ¹⁸O (5.8‰–7.2‰) values, and relatively cool crystallization temperatures (1125 to 1218 °C). These features support derivation of the lavas from highly refractory and cold SCLM. The relatively low partition coefficients of vanadium between olivine and whole rocks (0.019–0.025) indicate a high fO₂ for the Early Cretaceous basalts and their mantle sources (ΔQFM = +1.0 and ΔQFM = +1.5, respectively), and, consequently, an oxidized Late Mesozoic SCLM beneath the eastern NCC. The high degree of oxidation of the mantle was caused by the ingress of hydrous melts and/or fluids released from a subducting slab during the Phanerozoic. We propose that oxidization of the SCLM softened the mantle, which triggered the removal of the cratonic root beneath the eastern NCC. The results highlight the crucial role of oxidation state in craton stability.     

Early Cretaceous gabbroic complex from Yinan,Shandong Province: petrogenesis and mantle domains beneath the North China Craton

Sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO₂, low FeO and TiO₂ contents and a strong enrichment of light rare earth elements [(La/Yb)_n=11–50], but no Eu anomaly. They have low Nb/La (0.07–0.29), radiogenic ⁸⁷Sr/⁸⁶Sr (0.710) and unradiogenic _Nd(t) (–15 to –13). These crustal fingerprints cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (>20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low ⁸⁷Sr/⁸⁶Sr) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high ⁸⁷Sr/⁸⁶Sr) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros (127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.     

Water contents of the Cenozoic lithospheric mantle beneath the western part of the North China Craton: Peridotite xenolith constraints

Nominally anhydrous phases (clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol)) of peridotite xenoliths hosted by the Cenozoic basalts from Beishan (Hebei province), and Fansi (Shanxi province), Western part of the North China Craton (WNCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The H₂O contents (wt.) of cpx, opx and ol are 30–255 ppm, 14–95 ppm and ~ 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivine, pyroxenes (cpx and opx) largely preserve the H₂O content of their mantle source inferred from (1) the homogenous H₂O content within single pyroxene grains, and (2) equilibrium H₂O partitioning between cpx and opx. Based on mineral modes and assuming a partition coefficient of 10 for H₂O between cpx and ol, the recalculated whole-rock H₂O contents range from 6 to 42 ppm. In combination with previously reported data for other two localities (Hannuoba and Yangyuan from Hebei province), the H₂O contents of cpx, opx and whole-rock of peridotite xenoliths (43 samples) hosted by the WNCC Cenozoic basalts range from 30 to 654 ppm, 14 to 225 ppm, and 6 to 262 ppm respectively. The H₂O contents of the Cenozoic lithospheric mantle represented by peridotite xenoliths fall in a similar range for both WNCC and the eastern part of the NCC (Xia et al., 2010, Journal of Geophysical Research). Clearly, the Cenozoic lithospheric mantle of the NCC is dominated by much lower water content compared to the MORB source (50–250 ppm). The low H₂O content is not caused by oxidation of the mantle domain, and likely results from mantle reheating, possibly due to an upwelling asthenospheric flow during the late Mesozoic–early Cenozoic lithospheric thinning of the NCC. If so, the present NCC lithospheric mantle mostly represents relict ancient lithospheric mantle. Some newly accreted and cooled asthenospheric mantle may exist in localities close to deep fault.