Metamorphic convergence of the upper and lower plates of the Vincent thrust, San Gabriel Mountains, southern California, USA

The Vincent thrust of the San Gabriel Mountains, southern California, separates eugeoclinal Pelona Schist from overlying Precambrian to Mesozoic igneous and metamorphic rocks of North American continental affinity. The thrust is generally considered to be synmetamorphic because of similarity in structural orientations and mineral assemblages between the Pelona Schist and mylonites at the base of the upper plate. In this study, compositions of calcic amphibole and plagioclase in the upper plate and structurally high Pelona Schist were compared to further test this interpretation. Amphibole in the schist is mostly actinolite to actinolitic hornblende with high Na/Al ratio, indicating relatively high-P/low-T metamorphism. Individual grains are zoned, with concentrations of both Na and Al decreasing from cores to rims. Premylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization, this amphibole was replaced by actinolite to actinolitic hornblende with a similar range of Na and Al as amphibole rims in the Pelona Schist, but with slightly lower Na/Al ratio. This is consistent with the decrease of Na/Al up-section previously noted within the Pelona Schist of this area, and is considered to be the result of an inverted thermal gradient during thrusting. Convergence of composition between schist and upper plate also occurs for K and Ti contents of amphibole and An content of plagioclase. These features provide strong evidence that mylonitization of the upper plate is closely related in space and time to metamorphism of the Pelona Schist and therefore that the Vincent thrust is a remnant of the primary fault along which the Pelona Schist and correlative units were subducted beneath North America. Nonetheless, very fine-scale differences in amphibole composition between the schist and upper plate may indicate that metamorphic re-equilibration could not quite keep pace with movement on the fault.     

Occurrence and significance of a quartz–amphibole schist xenolith within a mafic microgranular enclave in the Xiangshan volcanic-intrusive complex,SE China

The Xiangshan volcanic-intrusive complex is composed of rhyolitic crystal tuffs, welded tuffs, rhyodacite, porphyroclastic rhyolitic lava, subvolcanic rocks such as granite porphyry, and late quartz monzonitic porphyry and lamprophyre dikes. We report the first occurrence of a quartz–amphibole schist (QAS) xenolith enclosed within a mafic microgranular enclave (MME) in the Xiangshan volcanic-intrusive complex. The mineralogy of this xenolith consists of amphibole, biotite, quartz, and minor plagioclase. Petrographic and mineral composition studies indicate that the protolith of this xenolith likely originated from the metamorphic basement beneath Xiangshan. The amphibole (actinolite and magnesiohorblende) has been partially replaced by orthopyroxene at 800–1000°C and by diopside at <700°C, according to mineral thermometers; this replacement process may have taken place after the xenolith was trapped by the mafic magma host (now an MME). Studies of the QAS xenolith provide new information on the emplacement history of the mafic magma. The peak metamorphic temperature for amphibole replaced by pyroxene is higher than the crystallization temperature of the subvolcanic magma, which indicates that the heat of pyroxene formation must have been provided by the engulfing mafic melt. This magma must have emplaced to crustal level and trapped the QAS as a xenolith and then injected into the felsic magma. We suggested that the hybridization processes for the major elements of the pristine mafic magma may have been contaminated by crustal rocks to form its present composition of MME before mafic magma injection. However, the hybridization process appears not to have been formed via a single-stage process because various types of MMEs are presented in the Mesozoic magmatic rocks of SE China.     

新疆拜城县波孜果尔碱性花岗岩体中角闪石与黑云母地球化学特征及其对成岩成矿的记录

角闪石和黑云母是含稀有稀土金属矿物的高分异碱性花岗岩中常见的暗色矿物。角闪石和黑云母对不同元素(尤其是Nb、Ta、Zr、REE等稀有稀土金属元素)的相容性特点使它们的主微量元素特征成为岩浆演化的"日志",对构建含矿岩浆的成岩成矿模型具有一定意义。本文针对新疆拜城波孜果尔碱性花岗岩体中的角闪石和黑云母,利用电子探针(EPMA)和激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)等分析技术,开展精细的矿物学研究,综合厘定Nb、Ta等元素在岩浆结晶过程中的富集和分配过程。波孜果尔岩体中的角闪石均为亚铁钠闪石,总体表现为较高的Na和Fe含量,而相对贫Ca(0.15 apfu)、Mg(0.06 apfu)和Ti(0.07 apfu),成分的均一性以及轻稀土元素的亏损特征暗示了角闪石较晚结晶且角闪石结晶前大量富轻稀土元素矿物结晶分离。根据共生关系黑云母可分为A、B两类,它们显示出不同的化学成分。A类黑云母具有相对较高的F含量(A类为3.81%～4.82%,B类为1.68%～3.65%)和SiO_2含量(A类为41.5%～43.3%,B类为35.9%～38.5%),相对较低的TFeO含量(A类为28.0%～31.4%,B类为32.2%～35.7%)和Al_2O_3含量(A类为6.45%～7.62%,B类为9.70%～11.00%)。黑云母与角闪石中氟的含量变化指示了氟在结晶过程中的富集、饱和过程以及氟饱和引起的稀有稀土金属矿物大量结晶现象。由于在主要矿物中的不相容性,成矿元素Nb、Ta在熔体中含量逐渐上升,但主要矿物及大部分副矿物中均高于全岩的Nb/Ta值产生了"Ta丢失"现象。结合前人研究,本文认为气热相带出与锆石的结晶分离两种机制共同作用导致Ta从熔体中分离。     

Alkaline rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India

The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, plug-like body approximately 12 km² area and is emplaced into the Precambrian gneissic terrain of the Karbi Anglong district of Assam. The host rocks, which are exposed in immediate vicinity of the intrusion, comprise granite gneiss, migmatite, granodiorite, amphibolite, pegmatite and quartz veins. The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite ± perovskite ± apatite rock, alkali pyroxenite, ijolite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocratic variants, phonolite, volcanic tuff, phosphatic rock and chert breccia. The magnetite ± perovskite ± apatite rock was generated as a cumulus phase owing to the partitioning of Ti, Fe at a shallow level magma chamber (not evolved DI = O1). The highly alkaline hydrous fluid activity indicated by the presence of strongly alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alkalic than those now found and represent a silica undersaturated ultramafic rock of carbonated olivine-poor nephelinite which splits with falling temperature into two immiscible fractions—one ultimately crystallises as alkali pyroxenite/ijolite and the other as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate and carbonate melts, produced through liquid immiscibility, during ascent generated into an array of lithotypes and also reaction with the country rocks by alkali emanations produced fenitic aureoles (nephelinisation process). Isotopic studies (δ¹⁸O and δ¹³C) on carbonatites of Samchampi have indicated that the δ¹³C of the source magma is related to contamination from recycled carbon.     

Experimentally Determined Phase Relations in Hydrous Peridotites to 6{middle dot}5 GPa and their Consequences on the Dynamics of Subduction Zones

Fluid-saturated subsolidus experiments from 2·0 to 6·5GPa, and from 680 to 800°C have been performed on threemodel peridotites in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O(NCFMASH). Amphibole and chlorite coexist up to 2·4 GPa,700°C. Chlorite persists to 4·2 GPa at 680°C.Starting from 4·8 GPa, 680°C a 10 Å phase structurereplaces chlorite in all compositions. The 10 Å phasestructure contains significant Al₂O₃ (up to 10·53 wt%) deviating from the MgO–SiO₂–H₂O 10 Å phase(MSH 10 Å phase). A mixed layered structure (chlorite–MSH10 Å phase) is proposed to account for aluminium observed.In the Tinaquillo lherzolite amphibole breakdown occurs viathe reaction Thermal stabilityof chlorite (chlorite + orthopyroxene = forsterite + garnet+ H₂O) is shifted towards lower temperatures, compared withthe system MASH. Furthermore, the chlorite thermal breakdownis also related to the degenerate reaction Chlorite and the Al-10 Å phase structurecontribute significantly to the water budget in subduction zonesin the depth range relevant for arc magmatism, whereas amphibole-relatedfluid release is restricted to the forearc region. Chloriteand Al-10 Å phase breakdowns might explain the occurrenceof a double seismic zone by dehydration embrittlement. KEY WORDS: amphibole; chlorite; high pressure; peridotites; subduction zones     

郯庐断裂(安徽段)两侧肥东群与霍邱群特征性岩石对比研究

郯庐断裂带(安徽段)内部物质组成十分复杂,现有研究认为郯庐内部以扬子板块为主体,对于是否存在西侧华北板块物质混入尚不清楚。为此本文选取了3个特征性样品展开了细致的岩相学研究、温压估算以及锆石U-Pb定年分析,分别为:霍邱群样品WS069-1(磁铁石榴角闪岩)以及样品WS070-1(石榴黑云母片岩);肥东群样品TF003-1(石榴黑云母片岩)。研究发现,霍邱群磁铁石榴角闪岩与肥东群磁铁石榴角闪岩的矿物组合及矿物主量元素特征基本一致。针对霍邱群石榴黑云母片岩与肥东群石榴黑云母片岩的峰期变质条件评价也相互匹配。本文在霍邱群仅发现了~3.2Ga、3.0Ga、2.80Ga、2.74Ga等四组反映源区结晶时代的碎屑锆石年龄,但结合前人对霍邱群和五河群的研究可以认为霍邱群的峰期变质年龄应为2.44Ga,这与肥东群磁铁石榴角闪岩和石榴黑云母片岩的峰期变质年龄2.47~2.41Ga吻合。因此肥东群和霍邱群相同特征性岩石磁铁石榴角闪岩从岩相学、变质P-T条件及变质时代等方面皆具可比性,从而推测肥东群磁铁石榴角闪岩和石榴黑云母片岩构造透镜体应为因郯庐断裂左旋走滑作用而卷入肥东群中的霍邱群岩片。     

Oriented growth of garnet by topotactic reactions and epitaxy in high‐pressure,mafic garnet granulite formed by dehydration melting of metastable hornblende‐gabbronorite (Jijal Complex,Kohistan Complex,north Pakistan)

Garnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉_Grt // [001]_Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.^?1 for the Grt‐in reaction and 0–5 kJ mol.^?1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.     

铜官山岩体矿物学-矿物化学特征：岩浆结晶动力学意义

本文对皖南官山岩体开展详细地显微镜观察鉴定,利用电子探针和LA-ICP-MS技术对岩浆岩典型矿物斜长石、角闪石和榍石进行了主量和微量元素测定。显微镜鉴定表明,铜官山岩体中存在着大量的岩浆不平衡结构：如斜长石和角闪石嵌晶结构以及针状磷灰石等。这些现象的存在表明铜官山岩体在形成过程中曾发生过一次或多次岩浆混合作用。电子探针分析结果显示,斜长石的成分环带是震荡环带,而大尺度的震荡环带可能代表了大规模的岩浆混合作用;角闪石成分TiO2-Al2O3图解、CaO/NaO2-Al2O3/TiO2图解和Mg-(Fe2++Fe3+)- LiNaKCa角闪石成因矿物族三角图解指示铜官山岩体中角闪石很可能为壳-幔混合成因。LA-ICP-MS技术对主要造岩矿物的微量和稀土元素分析表明,角闪石很可能为幔源或壳幔混合源,斜长石可能为不同分异程度岩浆的混合形成。本研究比较明确地反映了铜官山岩体的形成过程中岩浆来源和结晶动力学过程,即壳幔源区的混合交代作用,与前人通过元素-同位素手段获得的信息比较吻合。     

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group,Pitinga Mining District,Amazonian craton,Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO₂, FeO_t, K₂O, Rb, and alkalis; low TiO₂, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na₂O?+?K₂O averages of 8.8 wt.%, FeO_t/(FeO_t?+?MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb?+?Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO₂ and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.     

Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist

Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent-Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic-calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1^IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and Na^M4 to (A1^VI+ Fe³⁺+ Ti). These variations imply that structurally high rocks belong to a lower-pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.
Amphibole composition is dependent not only on structural position (i.e. P-T ), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe²⁺+ Mn) and Fe³⁺/Fe²⁺. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.
Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.