Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatífera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P–T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720–850 °C and 10–14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite–melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet.The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction–accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge.     

CRYSTALLOGRAPHIC PREFERRED ORIENTATION(CPO) OF ANISOTROPIC MINERALS IN THE LITHOSPHERE AND ITS SIGNIFICANCE TO THE STUDY OFLITHOSPHERE DYNAMICS

Seismic anisotropy has been widely used to constrain deformation and mantle flow within the upper mantle of the Earth's interior, and is mainly affected by crystallographic preferred orientation(CPO)of anisotropic mineral in lithosphere. Anisotropy of peridotites caused by deformation is the main source of seismic anisotropy in the upper mantle. Olivine is the most abundant and easily deformed mineral to form CPO in peridotite, thus the CPO of olivine controls seismic anisotropy in the upper mantle. Based on simple shear experiments and studies of natural peridotites deformation, several CPO types of olivine have been identified, including A, B, C, D, E and AG-type. Studies on the deformation of olivine have shown that the CPO of olivine is mainly related to stress, water content, temperature, pressure, partial melting and melt/fluid percolation. Most of the seismic anisotropy has been explained by the A-type olivine CPO in the upper mantle, which is commonly found in upper-mantle peridotites and produced by the simple shear in dry conditions. Previous studies showed that anisotropy was attributed to the CPO of mica and amphibole in the middle-lower crust. The comparison between mantle anisotropy calculated from mineral CPO and regional anisotropy deduced from geophysical methods is therefore particularly useful for interpreting the deformation mechanisms and geodynamic processes which affect the upper mantle in different tectonic units such as subduction system, continental rift and continental collision zone in the world. The paper summarizes the characteristics of CPO and anisotropy of major anisotropic minerals in the upper mantle. Taking the lithosphere mantle xenoliths in the southeastern Tibetan plateau as an example, we perform detailed studies on the microstructures and seismic anisotropy to better understand the deformation mechanisms and upper mantle anisotropy in this region. Results show that the CPO of olivine in peridotite xenoliths in southeastern Tibetan plateau are A-type and AG-type. The mechanisms proposed for the formation of AG-type are different from that for the A-type. Therefore, the occurrence of AG-type olivine CPO pattern suggests that this CPO may record a change in deformation mechanism and tectonic environment of the lithosphere in southeastern Tibetan plateau. Provided that the strong SKS(shear wave splitting)observed in southeastern Tibetan plateau results from lithosphere mantle, the lithosphere mantle in this region is expected to be at least 130km thick and characterized by vertical foliation. Considering that the thickness of lithosphere in southeastern Tibetan plateau is much less than 130km and the lithosphere mantle cannot explain the anisotropy measured by SKS, other anisotropy sources should be considered, such as anisotropy in the asthenosphere and the oriented melt pockets(MPO)in the upper mantle. Therefore, detailed study of CPO of anisotropic mineral is essential for constraining geophysical measurements and analyzing the dynamic process of the lithosphere reasonably.     

Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line.The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite.They commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is spinel-free.Amphibole is an essential constituent in the lherzolites.Mineral chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the dunites.Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and spinel.This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole.Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957
C,along a dynamic rift geotherm and shallower than the region where carbonate melts may occur.The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.     

吉林双辽地区橄榄岩包体中熔体-岩石作用特征及其对地幔交代作用的启示

吉林双辽地区古近纪玄武岩中一方辉橄榄岩包体记录了上地幔交代作用的信息。原生斜方辉石被交代成因的单斜辉石和橄榄石所围绕,或形成反应边结构,或斜方辉石残留在次生单斜辉石中。这些反应结构仅出现在尖晶石的周围。电子探针分析表明次生单斜辉石具有高Mg#、Cr#和CaO/Al2O3比值,次生橄榄石高Mg#、CaO和Cr2O3,被交代的尖晶石边部高Cr#、CaO。由于交代作用并未影响橄榄岩体系的Mg#,而且熔体-岩石反应结构指示Opx(斜方辉石) Sp(尖晶石) 熔体(Ⅰ)→Cpx(单斜辉石) Ol(橄榄石) 熔体(Ⅱ),结合实验以及文献资料,认为双辽地区的岩石圈地幔受到了硅酸盐熔体的交代。这种交代导致橄榄岩中斜方辉石逐渐减少,单斜辉石和橄榄石逐渐增加,从而使方辉橄榄岩渐变成易剥橄榄岩。这种交代现象可能发生在软流圈-岩石圈接触带上的熔-岩反应区,暗示了在古近纪双辽地区岩石圈减薄和软流圈上涌导致的软流圈-岩石圈的相互作用仍在继续。     

Continuation of the San Andreas fault system into the upper mantle: Evidence from spinel peridotite xenoliths in the Coyote Lake basalt, central California

The Coyote Lake basalt, located near the intersection of the Hayward and Calaveras faults in central California, contains spinel peridotite xenoliths from the mantle beneath the San Andreas fault system. Six upper mantle xenoliths were studied in detail by a combination of petrologic techniques. Temperature estimates, obtained from three two-pyroxene geothermometers and the Al-in-orthopyroxene geothermometer, indicate that the xenoliths equilibrated at 970–1100 °C. A thermal model was used to estimate the corresponding depth of equilibration for these xenoliths, resulting in depths between 38 and 43 km. The lattice preferred orientation of olivine measured in five of the xenolith samples show strong point distributions of olivine crystallographic axes suggesting that fabrics formed under high-temperature conditions. Calculated seismic anisotropy values indicate an average shear wave anisotropy of 6%, higher than the anisotropy calculated from xenoliths from other tectonic environments. Using this value, the anisotropic layer responsible for fault-parallel shear wave splitting in central California is less than 100 km thick. The strong fabric preserved in the xenoliths suggests that a mantle shear zone exists below the Calaveras fault to a depth of at least 40 km, and combining xenolith petrofabrics with shear wave splitting studies helps distinguish between different models for deformation at depth beneath the San Andrea fault system.     

Origin of fine‐grained peridotite xenoliths from Iraya volcano of Batan Island,Philippines: deserpentinization or metasomatism at the wedge mantle beneath an incipient arc?

Abstract Peridotite xenoliths from the subarc mantle, which have been rarely documented, are described from Iraya volcano of the Luzon arc, the Philippines, and are discussed in the context of wedge-mantle processes. They are mainly harzburgite, with subordinate dunite, and show various textures from weakly porphyroclastic (C-type) to extremely fine-grained equigranular (F-type). Textural characteristics indicate a transition from the former to the latter by recrystallization. The F-type peridotite has inclusion-rich fine-grained olivine and radially aggregated orthopyroxene, being quite different in texture from ordinary mantle-derived peridotites previously documented. Despite their strong textural contrast, the two types do not show any systematic difference in modal composition. The harzburgite of C-type has ordinary mantle peridotite mineralogy; olivine is mostly Fo91–92 and chromian spinel mostly has Cr#s (= Cr/[Cr + Al] atomic ratios) from 0.3 to 0.6. Olivine is slightly more Fe-rich (Fo89–91) and spinel is more enriched in Cr (the Cr#, 0.4–0.8) and Fe³⁺ in F-type peridotites than in C-type harzburgite. Orthopyroxene in F-type peridotites is relatively low in CaO (<1 wt%), Al₂O₃ (<2 wt%) and Cr₂O₃ (<0.4 wt%). The F-type peridotite was possibly formed from the C-type one by recrystallization including local dissolution and precipitation of orthopyroxene assisted by fluid (or melt) of subduction origin. Textural characteristics, however, indicate a deserpentinization origin from abyssal serpentinite of which protolith was a C-type peridotite. In this scenario the initial abyssal serpentinite was possibly dehydrated due to an initiation of magmatic activity beneath an incipient oceanic arc like Batan Island. The F-type peridotite is characteristic of the upper mantle of island arc, especially of incipient arc.     

揭示地质现象的本质与核心

青藏高原的形成与演化是举世瞩目的现代地学前缘 ,岩石圈结构及其演变对于探讨高原隆升及其资源环境效应具有重要意义。多年来 ,对于青藏高原岩石圈结构的认识主要基于地球物理探测结果和地质研究 ,尚未发现深达地幔的物质。我们在西昆仑地区康西瓦新生代火山岩中首次发现了以含金云母方辉橄榄岩为主的深源包体 ,其主要组成矿物为橄榄石和斜方辉石 ,含有少量单斜辉石、尖晶石和金云母。橄榄岩具有平衡粒状变晶结构 ,部分橄榄石颗粒仍保留有扭折带。电子探针分析结果表明 ,尖晶石的x(Cr) /x(Cr +Al)变化为 2 5%～ 8% ,属于尖晶石铬铁矿系列。根据CarrollWebb和Wood(1986 )提出的温压图解 ,橄榄岩形成压力大致为16 0 0MPa ,约相当于深度 56km。据此 ,认为康西瓦新生代玄武岩中产出的橄榄岩包体系岩浆活动过程中从岩石圈地幔携带上来的幔源物质 ,对于探讨青藏高原北部的岩石圈结构及其演化具有重要意义。     

宁镇地区成矿斑岩与铁镁质包体铜同位素研究

长江中下游地区成矿斑岩的成因及铜矿中铜的来源一直受到争议。寻找斑岩铜矿中铜的来源归根结底是限定其寄主岩的成因。本文拟通过测定宁镇地区一系列中生代岩体的成矿斑岩及其铁镁质包体的Cu同位素组成,来探究成矿斑岩中Cu的来源。测定结果显示,安基山闪长斑岩δ65Cu值为-0.13‰~+0.36‰;其辉长岩包体δ65Cu的值为+0.14‰~+0.19‰。铜山成矿斑岩δ65Cu值为+0.03‰~+0.36‰;其辉长岩包体δ65Cu的值为+0.10‰~+0.20‰。象山花岗闪长岩δ65Cu为+0.20‰±0.06‰;其辉长岩包体δ65Cu为+0.16‰±0.06‰。三个岩体成矿斑岩δ65Cu值为-0.13‰~+0.36‰,变化范围较大;而辉长岩包体δ65Cu值均在+0.1‰~+0.2‰之间,平均δ65Cu为+0.15‰±0.05‰,与地幔δ65Cu值(+0.07‰±0.10‰)吻合。结合前人对研究区成矿斑岩高Mg O、Mg#值,低放射性成因Pb同位素组成,以及古老下地壳来源的岩石具有较大的Cu同位素组成变化的认识,我们认为宁镇地区成矿斑岩及成矿物质主要为幔源岩浆和古老下地壳部分熔融岩浆混合成因。     

汉诺坝地幔捕虏体中富硅熔体的成因及其意义

近年来地幔捕虏体中富硅熔体的研究受到广泛关注,富硅熔体不仅具有多种产出状态,其成因来源也具有多样性。本文选取汉诺坝地区地幔捕虏体矿物中的熔体包裹体和斜方辉石反应边中的熔体玻璃为研究对象,探索富硅熔体在岩石圈地幔演化中的作用。通过详细的岩相学观察并结合电子探针分析发现,二者成因不同。熔体包裹体玻璃成分富硅(SiO₂ 61%~65%),相对贫Na₂O(1%~3%)、K₂O(<1%),富含挥发份(约3%~6%),为地幔交代成因;斜方辉石反应边玻璃富硅(SiO₂ 64%~67%)、碱(Na₂O 5%~7%,K₂O 6%~9%),几乎不含挥发份,是地幔捕虏体被寄主玄武岩浆快速携带上升至地表的过程中两者反应的产物。地幔交代作用产生的富硅熔体包裹体深刻影响了岩石圈地幔的地球化学性质;而玄武岩浆在上升过程中与岩石圈地幔捕虏体中斜方辉石的反应,导致了岩石圈地幔由富硅向贫硅转变,为华北克拉通的破坏提供了证据。     

Deformation microstructures of olivine and pyroxene in mantle xenoliths in Shanwang,eastern China,near the convergent plate margin,and implications for seismic anisotropy

Deformation microstructures, including lattice-preferred orientations (LPOs) of olivine, enstatite, and diopside, in mantle xenoliths at Shanwang, eastern China, were studied to understand the deformation mechanism and seismic anisotropy of the upper mantle. The Shanwang is located across the Tan-Lu fault zone, which was formed due to the collision between the Sino-Korean and South China cratons. All samples are spinel lherzolites and wehrlites, and LPOs of minerals were determined using scanning electron microscope/electron backscattered diffraction. We found two types of olivine LPO: type-B in spinel lherzolites and type-E in wehrlites. Enstatite showed two types of LPO (types BC and AC), and diopside showed four different types of LPO. Observations of strong LPOs and numerous dislocations in olivine suggest that samples showing both type-B and -E LPOs were deformed in dislocation creep. The seismic anisotropy of the P-wave was in the range of 2.2–11.6% for olivine, 1.2–2.3% for enstatite, and 2.1–6.4% for diopside. The maximum seismic anisotropy of the shear wave was in the range 1.93–7.53% for olivine, 1.53–2.46% for enstatite, and 1.81–6.57% for diopside. Furthermore, the thickness of the anisotropic layer was calculated for four geodynamic models to understand the origin of seismic anisotropy under the study area by using delay time from shear wave splitting, and S-wave velocity and anisotropy from mineral LPOs. We suggest that the seismic anisotropy under the study area can be most likely explained by two deformation modes that might have occurred at different times: one of deformed lherzolites with a type-B olivine LPO by lateral shear during/after the period of the Mesozoic continental collision between the Sino-Korean and South China cratons; and the other deformed the wehrlites with a type-E olivine LPO by horizontal extension during the period of change in absolute plate motion in relation to the westward-subducting Pacific plate.