Quench textures in pillow basalt from the Andaman-Nicobar Islands,Bay of Bengal,India

Tholeiitic pillow basalt from the South Andaman island, an integral part of the outer sedimentary arc of the Sunda-Burmese double chain arc system in the Bay of Bengal, is characterized by the occurrence of several morphologies of quenched crystals of plagioclase and pyroxene. Plagioclase shows a swallow tail, belt-buckle, rosette and closely spaced fan-spherulites pattern while pyroxene has elongate parallel chain, dendritic, spherulitic and finely ornamented feathery spherulitic habit. Most of these textures are identical to those reported from submarine basalts, lunar basalts, spinifex textured rocks and experimentally produced textures. The occurrence of these quench textures in the Andaman basalt suggests that they were formed by rapid cooling at 30–70°C/h in a submarine environment.     

内蒙古鄂温克族自治旗红花尔基斜长花岗岩的发现及锆石SHRIMP UPb测年结果

内蒙古鄂温克族自治旗红花尔基一带基岩出露情况差,地质工作程度低,以往工作所发现的侵入岩均划分进入中石炭世。通过对1∶5万区域地质调查工作,对发现的斜长花岗岩进行锆石SHRIMP U-Pb定年,所获得年龄为(172.2±1.3)Ma,更改为中侏罗世侵入体。推断红花尔基斜长花岗岩的形成是为中生代中、小块体之间的俯冲碰撞触发了新元古代加入地壳的亏损地幔物质熔融所形成。     

Growth of plagioclase rims around metastable kyanite during decompression of high‐pressure felsic granulites (Bohemian Massif)

Samples of high‐pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high‐pressure) mineral assemblage of garnet – kyanite – plagioclase – K‐feldspar – quartz ± biotite show well‐developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K‐feldspar. Kyanite is regarded as a relict of the high‐pressure metamorphic assemblage that became metastable during transition to a low‐pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An_32–25 at the contact with kyanite to An_20–19 at the contact with the matrix or to the perthitic K‐feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim‐forming reaction to provide the Al₂O₃ component for the growth of plagioclase, whereas in other cases no Al₂O₃ from kyanite was necessary. In a majority of examples, the necessary Al₂O₃ was supplied with CaO and Na₂O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ～ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ～ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K‐feldspar grains.     

Magma Chamber Process of Post-collisional Magmatism: Insight from Textural and Elemental Characteristics of Plagioclase from the Tatun Volcanic Group, Northern Taiwan Volcanic Zone

The Taiwan mountain belt, one of the youngest orogenies in the world, is caused by the collision of the Luzon arc with the Eurasian margin, which leads to post-collisional extension and magmatism in the Northern Taiwan Volcanic Zone (NTVZ). The magma chamber process in this region has not previously been elucidated in detail. In this paper, the textural and compositional features of plagioclase phenocrysts in basalt from the Tatun Volcanic Group (TTVG) were studied to restrict the dynamics of magma system. Results show that the magma melts in TTVG are mainly sourced from the underlying MORB-like mantle wedge but influenced by incorporation of subduction components, causing the elevated Sr/Y and Ba/Y ratios in magma melts. The subduction components are mainly transported in the form of sediment melt. The plagioclase phenocrysts in the TTVG volcanic rocks are generally coarsely core-sieved with a clear rim. The An contents in the rims of plagioclase are much lower than those of cores, and elevated FeO concentrations are detected in the plagioclase rims. We propose there exists a double-layer magma chamber in this region. The core of the plagioclase was crystalized in the deeper quiescent magma chamber (~21 km), which was subsequently partially dissolved during the ascent of magma melt under H2O-undersaturated condition, forming the typical coarsely sieved texture and synneusis. When this crystal-rich melt migrates into the shallower chamber, water saturation is reached and more sodic plagioclase formed as the rim of phenocryst. Due to the considerably higher fO2 in the shallow chamber than in the deeper one, and the distribution of Fe between plagioclase and melt positively correlates with fO2, the FeO content in the plagioclase rim elevates in conjunction with increasing fO2.     

宁镇中段岩浆带杂岩体的斜长石矿物学研究

运用光学显微镜、X射线衍射分析、扫描电镜微区能谱分析等手段,宁镇中段岩浆岩带安基山、伏牛山、西银坑、洪水宕、铜山、下蜀等杂岩体的斜长石斑晶的某些成因矿物学特点及其与岩浆演化分异特征的关系进行研究。结果表明:研究区中酸性岩体的斜长石主要为更-拉长石,韵律环带发育,形成经历了5个形成阶段,具有两种类型:一是核心为基性斜长石的正-反环带交互生长的斜长石;二是核心为中性斜长石的反-正环带交互发育的斜长石。研究还表明,斜长石各形成经历阶段、特别是晚期环境变化越和缓,岩浆分异程度越高。与其他各岩体相比较,西银坑、铜山岩体斜长石牌号明显偏高,反环带最发育,反映其岩浆同化混染更显著,而西银坑岩体岩浆结晶全程温度、K,Na,Ca组分相对活度波动频率更高,幅度更大。结合各岩体全岩及斜长石斑晶的成矿元素含量分析(Cu,Pb,Zn),认为岩浆结晶晚期分异充分,温度和K,Na,Ca组分相对活度波动和缓的铜山、安基山、伏牛山岩体是区内Cu多金属找矿的主要目标岩体。     

内蒙东南部早前寒武纪孔兹岩系中斜长石的成分环带

内蒙东南部早前寒武纪孔兹岩系中斜长石的Ca、Na、si、Al“等阳离子存在明显的成分环带。依据其成因机制可划分为生长环带和扩散环带。对不同成因的斜长石环带的成分,采用Ga—Pl—Al_2SiO_5(Sill)—Q压力计,配合Ga—Bi矿物对温度压力计估算,峰期前温度为570～620℃,压力1.0～1.2 GPa;峰期温度为750～850℃,压力0.75～0.85 GPa,峰期后温度为600～650℃,压力为0.5～0.62 GPa。可以确定本区早前寒武纪孔兹岩系变质作用演化为顺时针碰撞造山带型式的P—T—t轨迹。     

Peristerite compositions in quartzofeldspathic schists, Franz Josef-Fox Glacier Area, New Zealand

Plagioclase compositions vary from An_0.1–2.5 to An₃₂ with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An_20-24) and albite (An_0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An₁₂ and An₆ near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade. Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) P_H2O.     

西秦岭三叠纪酸性侵入岩中高An值斜长石的成因及其地质意义

研究I型花岗岩中再循环晶的成分和结构特征，对揭示岩浆系统的形成和演化历史以及壳源和幔源岩浆的相互作用具有重要的意义。本文以西秦岭北西段三叠纪过马营复式岩体内的不同造岩矿物为主要研究对象，通过对具有不同结构特征的斜长石"粗晶"和黑云母展开电子探针（EMPA）、LA-ICP-MS微量元素面扫描、原位Sr同位素分析，来探讨含高An斜长石的成因，示踪不同岩浆房端员的属性，约束岩浆演化过程并建立多级岩浆房模型。过马营复式岩体的岩性分为偏铝质花岗岩类和过铝质花岗闪长岩类，两类岩性中均存在An值呈突变的筛状结构斜长石，即高An（72~85）区与低An（20~55）区在同一颗粒中并存，其对应主、微量元素也存在截然变化。与低An区相比，高An区具有高Fe、Mg，高Ba的特征。高钙区斜长石可进一步分为两类，第1类高钙区斜长石（An₈₀-₈₅），和第2类高钙区斜长石（An₇₂-₇₈）。第1类高钙区比第2类高钙斜长石区具有更高的Ca、Al、Fe、Mg、Ba含量。本研究中斜长石晶体存在核_高An-边_低An与核_低An-幔_高An-边_低An两种不同类型的环带结构，表明其各自的生长过程有所不同。高An区与低An区之间An值跳跃式的变化与对应主、微量元素Fe、Mg、Sr、Ba含量特征均指示斜长石成分差异并非晶内扩散、动力学作用以及物理条件的变化造成的，更可能形成于开放的岩浆系统。本文认为斜长石粗晶为再循环晶，其内部的高An区形成于深部富H₂O玄武质岩浆，低An区形成于浅部酸性岩浆房（偏铝质与过铝质）。两种不同高钙区斜长石及其对应的主、微量元素差异指示它们很可能结晶于两批次不同的玄武质岩浆，其中第1类高钙区斜长石的母岩浆相对更富Fe、Mg、Ba。两种玄武质岩浆携带高An斜长石沿岩浆通道向上运移，上升过程熔蚀先前结晶的高An斜长石，将其带入浅部酸性岩浆房后与内部花岗质/花岗闪长质岩浆发生混合。混合之后的岩浆沿高An斜长石残晶边部继续结晶生长，同时高温玄武质岩浆的注入导致浅部岩浆房已结晶的低An斜长石被熔蚀形成筛状结构，随后玄武质岩浆、混合后岩浆依次沿其边部继续生长。我们认为两批次的玄武质岩浆体积较小并未对浅部酸性岩浆房成分造成大的影响。     

Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite

The inferred crystallization history of the troctolitic LowerZone of the Kiglapait Intrusion in Labrador is tested by meltingmineral mixtures from the intrusion, made to yield the observedcrystal compositions on the cotectic trace of liquid, plagioclase,and olivine. Melting experiments were made in a piston-cylinderapparatus, using graphite capsules at 5 kbar. Lower Zone assemblagescrystallized from 1245°C, 5% normative augite in the liquid,to 1203°C, 24% normative augite in the liquid at saturationwith augite crystals. This transit is consistent with modaldata and the large volume of the Lower Zone. The 1245°Ccotectic composition matches the average Inner Border Zone composition.Quenched troctolitic liquid from the Upper Border Zone, andothers from nearby Newark Island, plot on or near our experimentalcotectic, supporting a common fractionation history. Olivine–plagioclaseintergrowths from cotectic troctolitic melt show mosaic texturesreflecting the differing barriers to nucleation of these twophases. The linear partitioning of X_Ab in plagioclase–meltyields an intercept constant K_D = 0·524 for these maficmelts. Observed subsolidus exchange of Ca between plagioclaseand olivine elucidates the loss of Ca from plutonic olivines.The bulk composition of the intrusion is revised downward inFo and An. KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioningAbbreviations: AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, AUG: (Oxygen) Normative components; Ap, Aug, Ilm, Ol, Pl: Phases; Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers; BSE: Back-scattered electron; CaTs: Calcium Tschermak's component, CaAlAlSiO₆; D: Partition coefficient; f: Fugacity; F_L: Fraction of the system present as liquid = 1 – (PCS/100); FMQ: Fayalite = magnetite + quartz buffer; IBZ: Inner Border Zone; IW: Iron = wüstite buffer; kbar: kilobar, 10⁸ pascal; K_D: Exchange coefficient; KI: Kiglapait Intrusion; L: Liquid phase; LLD: Liquid line of descent; Ma: Mega-annum, age; Myr: Mega-year, time; OLHY: Normative OL + HY; OLRAT: The ratio OLHY/(OLHY + AUG); P: Pressure; P: Phosphorus; PCS: Percent solidified (volume); SMAR: South Margin average composition; T: Temperature, °C; UBZ: Upper Border Zone; WM: Wüstite = magnetite buffer; Wo: Wollastonite component of pyroxene; X: Mole fraction; X_Mg: Molar ratio Mg/(Mg + Fe²⁺); , X_Mg(0): Initial X_Mg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis     

Localized rheological weakening by grain-size reduction during lithospheric extension

Grain-size reduction may be a possible mechanism for the origin of localized deformation in the ductile regime. I investigated the effects of grain-size reduction due to dynamic recrystallization, cataclasis, and syntectonic metamorphic reaction on the stress envelope in the lithospheric mantle during extension by using a simple one-dimensional model. In this model, the lithosphere extends uniformly with a constant strain rate, and a fall in rock strength appears as a decrease in stress. Because grain-size distribution at the onset of extension is unknown, I regarded the steady state grain-size due to dynamic recrystallization as the initial size. Then, I evaluated the maximum effects of grain-size reduction by dynamic recrystallization during extension, and consequently examined the effects of grain-size reduction by cataclasis and metamorphic reaction under conditions when dynamic recrystallization occurs significantly. I find that it is difficult to bring about localized rheological weakening by grain-size reduction owing to dynamic recrystallization. In contrast, grain-size reduction by cataclasis can cause localized weakening during extension. There is a wide-ranging rate of grain-size reduction by means of cataclasis that causes localized weakening just below the Moho. I specified the reaction from spinel-lherzolite to plagioclase-lherzolite that plays a role in grain-size reduction by syntectonic metamorphism. The reaction occurs at depths less than 35 km, which is independent of the initial thermal state of the lithosphere. Localized rheological weakening can occur if the following conditions are satisfied: (1) grain-size before the reaction is greater than 0.7 mm under dry conditions and greater than 0.5 mm under wet conditions, and it decreases down to those values by the reaction; (2) grain-size decreases down to less than initial grain-size, when the dominant deformation mechanism is GSS creep at the onset of extension. It is also noted that dry conditions are more favourable for localized weakening.