Physical evolution of Grande Ronde Basalt magmas, Columbia River Basalt Group, north-western USA

Summary In this paper we present what is, to the best of our knowledge, the first comprehensive study of clinopyroxenes and plagioclases contained in the flows of the Grande Ronde Basalt member of the Columbia River Basalt Group (northwestern USA). The rocks have MgO(wt%)<6%, and trace amounts of Cr and Ni. About 56% of extracted solid containing normative clinopyroxene and plagioclase explains the liquid line of descent from the more mafic sample (MgO wt%=5.89) to the most evolved. The most ubiquitous phases in the basalts are plagioclase and augite. Ilmenite and magnetite are accessories in all rocks. Olivine is present in small amount only in one sample (RT 89-7). Based on principles of Ca–Na plagioclase–liquid exchange, estimates of pre-eruptive magmatic water are < 2.4wt%. From clinopyroxene–liquid equilibria, calculated pressures and temperatures of ascending magmas are between 1atm and 0.617GPa, and 1068°C and 1166°C, respectively. Compositions of magnetite–ilmenite pairs and olivine–clinopyroxene–oxide assemblages yield post-eruptive oxygen fugacities of NNO=–1.923, and one pre-eruptive value of NNO=– 2.455. A simple model of asthenospheric melting and magma ponding in the lower crust fits the physical parameters.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-003-0017-1     

Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks,South-Central Washington

More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger:

1. a)
   the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene).     
   
   

Crystal Fractionation and Recharge (RFC) in the American Bar Flows of the Imnaha Basalt, Columbia River Basalt Group

Variable isotope and incompatible element ratios require multiplesources for the Columbia River Basalt Group (CRBG) as a whole,but smaller, stratigraphically coherent, groups of flows withinthe CRBG show much smaller variations in these ratios and appearto be derived from a single source. One such group of flows,the American Bar (AB) subgroup of the Imnaha Basalt, is examinedin detail. Sixty-nine samples from the six major AB flows havebeen reanalyzed for 23 major and trace elements in order tofurther constrain the processes responsible for the variationsbetween their compositions. It is shown that the compositional differences between the ABflows include the compatible behavior of Sr, which requiresthe participation of plagioclase, and can be accommodated ina model which fractionates a crystal assemblage of plagioclase,clinopyroxene, and orthopyroxene (16:10:4). Apparent differencesin the degree of fractionation between major elements ({smalltilde}30%) and trace elements ({small tilde}50%) are mainlyexplicable in terms of recharge accompanying the crystal fractionation(RFC), but some ambiguity in the enrichment factors for Y, P,and Zr (but not for Nb) remains unexplained and permits thepossibility of some crustal assimilation. The fractionation assemblage (plagioclase+augite?orthopyroxene)differs from the observed phenocryst suite (plagioclase+olivine+augite)and, in the absence of critical experimental data, is assumedto represent a higher pressure assemblage stable near the crust/mantleboundary. The physical model which has been developed to explainthe AB eruption entails a large magma reservoir in the lowercrust immediately above the crust/mantle boundary in which fractionationand recharge accompanied periodic eruption, probably withoutsignificant crustal assimilation either within the magma reservoiror during the rapid expulsion of magma to the surface. Chemical,isotopic, and mineralogic differences between coherent flowsubgroups (Rock Creek, American Bar, and the lower flows ofGrande Ronde Basalt, for example) are abrupt. They include changesin isotope and incompatible element ratios as well as changesin SiO₂ and K concentrations which require magma derivationfrom a different combination of sources.     

Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, ⁸⁷Sr/⁸⁶Sr=0.70307–0.70371, _Nd=+7.7-+4.8, ¹⁸O=+5.6±6.1, and ²⁰⁶Pb/²⁰⁴Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low ⁸⁷Sr/⁸⁶Sr, high _Nd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like ¹⁸O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher ⁸⁷Sr/ ⁸⁶Sr and ¹⁸O, lower _Nd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K₂O/TiO₂, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.Deceased     

Chloritoid-bearing pelitic rocks of the Horsethief Creek Group,Southeastern British Columbia

Aluminous pelitic rocks of the Late Precambrian Horsethief Creek Group of southeastern British Columbia contain the assemblage chloritoidmuscovite-paragonite-quartz-chlorite (biotite zone). Additional members of the assemblage may include graphite, Fe-Mg carbonate, rutile, ilmenite and pyrite. No albite was detected. Lower grade pelitic rocks (chlorite zone) contain muscovite-chlorite and rare paragonite.Chloritoids from carbonate-free assemblages show a narrow range of composition (85±5 mol % Fe-chtd) and most porphyroblasts are zoned with higher Mn in cores and higher Mg in rims. For eight chloritoid-chlorite pairs, K _D = (Mg/Fe chtd/Mg/Fe chl) = 0.188±0.0234.Correlation of these mineral assemblages with experimental and computed phase equilibria and oxygen isotope temperatures suggest a minimum pressure near 4.5 Kbar, a minimum temperature near 335 ° C and an upper limit on temperature near 460 ° C. Variation in X _{CO 2} content of fluids attending metamorphism is inferred from the alternate appearance of either Fe-Mg carbonate + rutile or ilmenite-bearing assemblages. The assemblage paragonite-chloritoid-quartz-Fe-Mg carbonate-rutile is inferred to be stable at a T near 360 ° C, an X _{CO 2} near 0.9 and P near 5 Kbar.     

Multiple paleosols of the late Albian Boulder Creek Formation, British Columbia, Canada

Fifteen lithified paleosols, closely spaced in vertical sequence, occur in the top 90 m of the late Albian Boulder Creek Formation in the foothills of northeastern British Columbia, Canada. The paleosols have well-developed profiles 0·5 to 1·5 m thick, including A, B and C horizons. The paleosols are characterized by their grey colour, cutans, vertical roots, peds, spherulitic siderite and absence of sedimentary structures. The paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events. These unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain. The climate was humid to subhumid. Overall, the environment in which these soils developed was of low relief and subject to little erosion. The water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris. Cumulatively, the paleosols in this interval may represent 150000 yr of non-deposition.     

Isotopic and Geochemical Constraints on the Origin and Evolution of the Columbia River Basalt

New major and trace element data on over 70 samples are combinedwith a wider knowledge of the regional stratigraphy, and ofthe tectonic evolution of the boundary between the ColumbiaPlateau and the northern margin of the Basin and Range province,to distinguish three subgroups within the Columbia River BasaltGroup (CRBG): the Picture Gorge Basalt; the main sequence ofColumbia River flood basalts, here named the Clarkston Basalt;and the Saddle Mountains Basalt. The subgroups are characterizedby different incompatible element and Sr-, Nd-, and Pb-isotoperatios, and they are interpreted in terms of different sourceregions mobilized under different tectonic conditions. The majordifferences between the subgroups are consistent with partialmelting processes in the upper mantle, and it is argued thatthey reflect previous partial melting episodes which resultedin source regions that were variably enriched and depleted inincompatible elements. The major variations within the PictureGorge and Clarkston Basalt subgroups include increases in theabundances of large ion lithophile elements (LILE) and increasesin the ratios of LILE/high field strength elements (HFSE) whichare interpreted as the addition of a lithospheric/subduction-relatedcomponent. The Picture Gorge Basalt has a depleted isotopic and chemicalsignature on which is superimposed an enrichment of LILE toproduce a trace element pattern similar to that of other 17–0-Mabasalts erupted south of the Olympic Wallowa Lineament. Thispattern is characteristic of volcanism associated with the Basinand Range extensional province, and others have attributed itto a source component derived from an enriched subcontinentallithospheric mantle (SCLM). Of the Clarkston Basalts, the Imnaha and Grande Ronde Basaltsform chemical and isotopic arrays which indicate mixing of componentsfrom two distinct source regions. One had high ratios of LILE/HFSEand light rare earth elements (LREE)/HFSE, and as these arenot common in oceanic basalts, this component is thought tohave been derived from the continental mantle lithosphere. Itsisotope ratios are more enriched (older?) than those of thePicture Gorge Basalt, and its Rb/Sr ratios are much higher thanthose in its source rocks, consistent with preferential mobilizationof LILE at the time of magmatism. The second component was derivedfrom an asthenospheric source similar to that of Hawaii basaltsand is most obviously attributed to mantle plume activity. Basaltsof the Eckler Mountain and Wanapum Formations (smaller, separateformations of the Clarkston Basalt as redefined in this paper)fit this mixing model less well and may represent mixing betweenmore than two components. Flows of the third CRBG subgroup,the Saddle Mountains Basalt, also carry a lithospheric geochemicalsignature and have long been recognized as having more radiogenicisotopic signatures than the other two subgroups. Thus, SaddleMountains flows appear to require a lithospheric source enrichedin LILE at an even earlier time, and we concur with other workersthat the isotopic and chemical evidence implies their derivationfrom subcontinental lithospheric mantle enriched at {small tilde}2000Ma. Within each subgroup, the chemical effects of partial melting,fractional crystallization, and magma mixing processes can allbe distinguished within particular flow sequences. In the ImnahaBasalt variable degrees of partial melting during the generationof the CRBG magmas, and gabbro fractionation within the lowercrust, played major roles in their evolution. In the GrandeRonde Basalt fractional crystallization appears to be restrictedto >10%. The chemical and isotopic data for each CRBG subgroup, and thedifferent sources which those data imply, can be accommodatedin a tectonic model which includes the passing of the Yellowstonehotspot south of the center of the CRBG eruption before significantBasin and Range extension had moved north of the Brothers Faultzone at 15 Ma.     

岩浆混合作用的类型、标志、机制、模式及其与成矿的关系——以新疆北部为例

新疆北部广泛发育具有岩浆混合特征的岩体,岩体类型主要有钙碱性花岗岩、碱长花岗岩和碱性花岗岩,其中以第一类最为发育;岩体中的暗色包体类型以闪长质为主,成分可从闪长岩、石英闪长岩、石英二长闪长岩,过渡到花岗闪长岩,部分地区可发育辉长质、安山玄武质、安山玢岩质包体;有些地区还发育有岩浆混合岩.该区岩浆混合作用主要形成于四个大地构造演化阶段,即D3-C1交界的洋-陆俯冲期、C1末-C2初的碰撞期、C2-P的后碰撞期和T1的板内期.岩浆混合作用的判别标志有岩体宏观标志(近等轴状)、野外宏观标志(暗色微粒包体发育,有时可见岩浆混合岩)、显微标志(淬冷和矿物不平衡结构)、岩石地球化学标志(线性过渡特征)、年龄(一致)标志等.该区的岩浆混合作用是多级次的,其深部岩浆源区的混合表现为壳幔相互混合作用;混合作用表现出多阶段性,以基性岩浆注入到酸性岩浆为主要方式,由物理混合到化学混合渐变过渡.该区岩浆混合作用与斑岩铜钼金矿床密切相关,其成矿关系模式可概括为:基性岩浆与花岗质岩浆混合产生混合岩浆,经过进一步分异演化形成了含矿岩浆;与岩浆混合作用有关的斑岩铜钼矿的找矿标志为:岩浆混合作用强烈的大岩体内或附近晚期补充侵入的小型斑岩体具有找矿潜力.     

东昆仑造山带晚三叠世岩浆混合作用：以和勒冈希里克特花岗闪长岩体为例

东昆仑造山带晚华力西期-印支期花岗质岩石中广泛发育暗色微粒包体.本文以东昆仑东段和勒冈希里克特花岗闪长岩体为例,对暗色微粒包体及其寄主岩进行了详细研究.包体的野外产出特征、形态、结构构造和矿物学特征表明,他们是基性岩浆进入中酸性岩浆快速冷凝结晶的产物,和寄主花岗岩有着相似的稀土元素配分模式,显示轻稀土富集,重稀土亏损,轻重稀土元素分馏明显的特征,微量元素蛛网图也具有明显的相似性,富集大离子亲石元素,亏损高场强元素,反映了岩浆混合作用的特征;LA-ICP-MS锆石U-Pb同位素年代学研究显示,暗色微粒包体的形成年龄为(224.9±4.1)Ma,与寄主岩的同位素年龄值(225±5)Ma在误差范围内一致,表明了在晚三叠世期间东昆仑地区存在着后碰撞阶段壳-幔岩浆混合作用.     

小兴安岭晚石炭世花岗岩岩浆混合作用的岩相学证据及其地质意义

小兴安岭晚石炭世花岗岩具有明显的岩浆混合特征。岩体中暗色岩浆包体发育,主要为细粒闪长质岩浆包体,包体形态多样、大小不一,与寄主岩石呈截然、模糊或过渡关系。包体的矿物组合明显不平衡,如出现了寄主岩石中的碱性长石捕虏晶,有时可见其具暗色矿物镶边,发育针状磷灰石。这表明小兴安岭晚石炭世花岗岩的岩浆混合表现为基性岩浆和酸性岩浆的混合。这为探讨这些花岗岩的成因提供了岩石学依据,同时也暗示晚古生代佳木斯—松嫩古陆可能发生过地壳的垂向生长。     

大兴安岭北段上侏罗统哈日陶勒盖玄武岩的厘定及其地质意义

大兴安岭北段西部新巴尔虎右旗地区新厘定出非正式填图单元——上侏罗统哈日陶勒盖玄武岩,其岩石组合为灰黑色、深绿灰色、紫褐色玄武岩以及气孔杏仁状玄武岩、碱性橄榄玄武岩、玄武粗安岩等。锆石SHRIMP U Pb定年法测得其年龄数据为(1467±31) Ma,时代为晚侏罗世。岩石地球化学分析显示,哈日陶勒盖玄武岩属高钾钙碱性系列,岩石中Sr含量低,富集大离子亲石元素Rb、K、Ba,亏损高场强元素Nb、Ti,稀土元素含量总体较高,轻重稀土分馏明显,略具有铕负异常。上侏罗统哈日陶勒盖玄武岩的厘定说明,大兴安岭地区存在晚侏罗世基性火山岩浆活动事件。通过区域地质调查和地层对比,上侏罗统哈日陶勒盖玄武岩与上侏罗统伊列克得组原始定义相符,二者实则为同物异名,建议恢复上侏罗统伊列克得组。这一认识对大兴安岭中生代构造岩浆活动的研究及中生代火山岩地层划分对比具有重要地质意义。     

Basalt magma genesis and fractionation in collision- and extension-related provinces: A comparison between eastern, central and western Anatolia

Experimental studies of synthetic and natural basalt systems suggest that conditions of magma genesis and fractionation depend fundamentally on mantle temperatures and lithospheric stress fields. In general, compressional settings are more conducive to polybaric fractionation than extensional settings and in this regard, the Anatolian magmatic province offers a natural laboratory for comparing near-coeval basalt eruptions as a function of regional tectonics — compressional (collision-related) régimes dominating in eastern Anatolia and extensional tectonics characterizing a western province related to Aegean Sea opening. Projection of Plio-Quaternary basalt normative compositions from the Western Anatolia Extensional Province (WAEP), the Central Anatolian ‘Ova’ Province (CAOP), and Eastern Anatolia Compressional Province (EACP) are projected onto Ol–Ne–Cpx and Pl–Cpx–Ol planes in the simplified basalt system (Ne–Cpx–Ol–Qz), each showing distinctive liquid lines of descent. WAEP basalts are mostly constrained by low-pressure (< 0.5 GPa) cotectics while CAOP and EACP compositions conform to moderate and/or high-pressure (0.8–3.0 GPa) cotectics. Overall, a quasi-linear shift from moderate and/or high-pressure to low-pressure equilibria matches the westward transgression from compressional east Anatolia to the extensional west Anatolian–Aegean region. Comparison of their respective primary (mantle-equilibrated) magmas–simulated by normalizing their compositions to MgO = 15 wt.% (Mg-15)–with parameterized anhydrous and H₂O-undersaturated experimental melts suggests they segregated from spinel- to garnet-lherzolite mantle facies at pressures between c. 2 and 3 GPa (c. 70–100 km depth) under H₂O-undersaturated conditions. Interpolated potential temperatures (T_p) and lithospheric stretching factors (β) range as follows: (1) eastern Anatolian basalts associated with the Arabian foreland show T_p varying between 1250 and 1400 °C (except for the Karacalidag alkali basalts, south of the Bitlis–Zagros fracture zone, for which T_p ranges up to 1450 °C), for β values of 1.2–1.8. T_p values for central Anatolia (e.g. Sivas) range between 1300 and 1375 °C (except for Karapinar, Egrikuyu and Hasandag, which show < 1150 °C), and β values of 1.3–1.4. For western Anatolian basalts, T_p range mostly between 1250 and 1330 °C, except for a single value for Canakkale of 1400 °C and Kula sample showing T_p < 1200 °C, and β values of 1.3–2.0. Variation of these conditions is as great or greater than that between provinces, although there are clearly significant constraints on the inferred polybaric to low-pressure isobaric fractionation régimes. Covariation of total FeO, TiO₂, La/Yb, Ce/Sm, Zr/Y and Zr/Nb reflects small but significant differences in bulk composition and ambient melt fraction while the covariance of Ce/Sm and Sm/Yb is consistent with the segregation of primitive melts at the spinel- to garnet-lherzolite transition.     

东昆仑东段三叠纪岩浆混合作用:以香加南山花岗岩基为例

香加南山花岗岩基位于东昆仑造山带东段,岩基主要岩石类型为花岗闪长岩。千瓦大桥-加鲁河一带花岗岩体为香加南山岩基的重要组成部分。香加南山花岗岩基含大量暗色微粒包体,包体中捕掳晶丰富。千瓦大桥-加鲁河一带花岗岩体寄主岩中斜长石和暗色微粒包体中捕掳晶斜长石具正常环带,An值震荡变化,角闪石和黑云母Mg O含量和Mg#值较低,具壳源特征;暗色微粒包体中基质斜长石具核边结构,核部和边部An值存在间断,角闪石和黑云母Mg O含量和Mg#值较高,具幔源特征。LA-ICP-MS锆石U-Pb同位素定年结果显示千瓦大桥花岗闪长岩、暗色微粒包体和加鲁河辉长岩的结晶年龄分别为251.0±1.9Ma、252.8±3.0Ma和221.4±3.3Ma。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具较低的Mg#和Nb/Ta比值;从千瓦大桥到加鲁河花岗闪长岩呈现出由准铝质中钾钙碱性系列向准铝-弱过铝质中钾-高钾钙碱性系列演化;暗色微粒包体和加鲁河辉长岩轻重稀土元素分异程度相对较低,具较高的Mg#和Nb/Ta比值。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩分别为古特提斯演化俯冲阶段和后碰撞阶段幔源岩浆底侵新生地壳使其部分熔融产物。镁铁质岩浆注入长英质岩浆的混合作用形成了暗色微粒包体。岩浆混合过程中,如果岩浆不完全混合,混合岩浆中混入物质除了长英质岩浆的残留岩浆和捕掳晶,还应该有镁铁质岩浆与长英质岩浆之间的元素梯度差导致的物质扩散;如果岩浆为近完全混合,混合岩浆近似为镁铁质岩浆和长英质岩浆以一定比例二元混合。东昆仑东段晚古生代-早中生代幔源岩浆对花岗质岩浆的影响是一个持续的过程,从俯冲阶段早期流体交代地幔熔融,到俯冲阶段后期板片断离,然后同碰撞阶段板片断离的持续影响,再到后碰撞阶段加厚地壳的拆沉作用,由于地球动力学体制不同,导致幔源岩浆影响的大小和特征不同。     

The vertical distribution of minerals in coal zones A,B, C,D, Hat Creek,British Columbia

The four superimposed coal zones A, B, C, D lie in the Hat Creek graben in folded and faulted Eocene strata. 76 consecutive samples from DDH 75-106 were analysed by XRD and examined in polished section. Kaolinite and quartz are the major minerals with kaolinite increasing with depth. Some upper samples grade into halloysite. Montmorillonite-bentonite is virtually absent. Illite, mostly in Zone A, varies irregularly with depth, appears to be antipathetic to dolomite and, between 160 and 306 m, to be associated with tonstein. Pyrite occurs throughout the core. Only above 358 m is it detected by XRD. Pyrite and siderite occur in polished section but in XRD appear to be unrelated.Zone C shows a marked deficiency in mineral varieties. Cristobalite is found throughout Zone A and in the lower half of D, major gypsum in the upper part of A and in the lower part of D. No correlation exists between minerals and palynomorphs.     

Partial resolution of sources of n-alkanes in the saline portion of the Parachute Creek Member, Green River Formation (Piceance Creek Basin, Colorado)

Systematic variations in the 13C contents of individual extractable n-alkanes (C16-C29) can be modelled quantitatively and interpreted as indicating contributions from at least five distinct sources. These appear to be cyanobacterial (C16-C18, delta 13C = -37% vs PDB), phytoplanktonic (C16-C23, delta = -32%), chemoautotrophic bacterial (C20-C29, delta = -38%), phytoplanktonic or heterotrophic bacterial (C20-C29, delta = -30%), and vascular plants (C23-C29, delta = -29%). Hydrous pyrolysis of related kerogens yields large quantities of additional n-alkanes with different and much more uniform delta values. The latter materials are apparently derived from the thermolysis of aliphatic biopolymers whose presence in the Green River Oil Shale has been recognized visually.     

Geometry and pressure-temperature significance of the kyanite-sillimanite isograd in the Mica Creek area,British Columbia

Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al ₂SiO₅-quartz) does not provide a fine enough resolution to detect these predicted differences.     

同源岩浆不同期次之间混合产生的暗色包体——以北拉萨地块中部晚白垩世桑心日岩体为例

本文在研究西藏北拉萨块体中段桑心日岩体中的暗色包体时发现了一种具有特殊岩石成因的暗色包体。暗色包体呈椭球状,在暗色包体和寄主岩的接触面上通常形成一个明显的可能由风化作用造成的间隙面。暗色包体为二长玢岩-花岗闪长玢岩,寄主岩为花岗岩,暗色包体明显较寄主岩更基性,更富Na_2O、CaO、MgO和Fe_2O_3~T。暗色包体和寄主岩具有明显不同的稀土元素特征,暗色包体的稀土元素含量变化较大,最基性样品具有最高的稀土含量,随着基性程度的降低稀土元素含量明显下降。随着岩浆的进一步演化,岩浆向花岗岩方向演化,稀土含量又逐步升高。寄主岩和暗色包体具有基本一致的微量元素组成,具有典型的弧岩浆岩的特征,富集Rb、Cs、K等大离子亲石元素和Th、U,亏损Nb、Ta、Ti等高场强元素。此外,暗色包体和寄主岩具有明显的Ba、Sr的负异常。暗色包体成岩年龄为75. 6±1. 2Ma,寄主岩的成岩年龄为71. 8±0. 6Ma,暗色包体成岩年龄较寄主岩早约4Myr。两者具有一致的锆石原位Lu-Hf同位素特征。综合以上岩相学、年代学、元素地球化学和同位素地球化学证据,我们认为桑心日暗色包体和寄主岩来源于同源母岩浆,初始岩浆在母岩浆房中经历了不同程度的含钾角闪石结晶分离作用,并沿早期较弱的构造裂隙侵入到地壳的某一层位,随着构造活动进一步加剧,经过进一步分异母岩浆大规模上侵,并将早先侵位处于半塑性状态的暗色包体侵吞、裹挟至近地表。桑心日暗色包体最可能的成因模式可以解释为同源岩浆不同期次间的物理混合。     

贺兰山以南中奥陶统香山群徐家圈组古水流分析

贺兰山以南中奥陶统香山群为一套遭受轻微区域变质的陆源碎屑岩,并夹有少量碳酸盐岩和硅质岩,属于深海浊流沉积。其下部徐家圈组由灰绿、黄绿色轻变质细粒砂岩、钙质砂岩及粉砂岩和页岩组成,沉积环境为深水斜坡环境,并在该组发现了内波、内潮汐沉积,主要表现为双向交错层理。本文通过对徐家圈组指向沉积构造所显示的古水流资料进行研究,综合分析了香山群徐家圈组沉积时浊流方向和古斜坡方向,并利用古水流资料探讨了内波流和内潮汐流的方向。结果表明,槽模古水流方向从北向南略呈扇形发散状,代表了沉积时的浊流方向;交错层理古水流方向主要分散在NWW—NE之间,可以代表内波流和内潮汐流方向。对古水流平面分布特征进行综合分析,可以判断出沉积时区域斜坡方向与浊流方向基本一致,大致为SSW向,沉积时内波和内潮汐的传播方向则大致为NW向。     

东秦岭丹凤岩群的形成时代和构造属性

东秦岭丹凤岩群位于秦岭商丹构造带上，主体形成于新元古代（1000Ma-800Ma)，其中变质基性火册岩的岩石地球化学特征表明为岛弧型火山岩系，而非蛇绿岩，其形成于秦岭造山带新元古代主造山期板块构造体制下的活动陆缘古岛弧构造环境。