Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U–Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U–Pb apatite ages are 2.4–2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz–biotite–plagioclase–garnet–staurolite–kyanite in the pelitic schists; (b) quartz–biotite–plagioclase–low-Ca amphiboles–kyanite in Mg–Al-rich schists, and locally (c) hornblende–plagioclase–garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg–Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from >7 kbar to <3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78–1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.     

Sm–Nd garnet dating of polyphase metamorphism: northern Coast Mountains, south-eastern Alaska, USA

Sm–Nd ages of garnet from the northern Coast Mountains of south-eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole-rock Sm–Nd ages for a sillimanite-zone amphibolite (Taku Inlet) and a biotite-zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 Ma, respectively. Garnet core–whole-rock (80±9 Ma), core–matrix (84±9 Ma), rim–whole-rock (59±4 Ma) and rim–matrix (62±4 Ma) ages were obtained from a sample collected 200  m west of a Palaeocene Coast plutonic–metamorphic complex sill-like pluton that separates medium-grade metamorphic rocks from high-grade metamorphic rocks and voluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high-grade metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet ( c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre-71 Ma garnet-zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20  Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

Effects of fluid production on fluid flow during regional and contact metamorphism

Numerical and analytical models of fluid flow that account for fluid production during prograde regional and contact metamorphism show that expulsion of metamorphic fluids dominates the convective flux when crustal permeabilities are less than 0.1–100 μD, depending primarily on the rate of fluid production. When this is the case, fluid circulation is limited or prevented, fluid pressures are elevated above hydrostatic values, and flow throughout most of the model is up and away from the region of maximum fluid production. Fluid circulation is predicted to occur where permeability is high, in dry rocks, or after rates of fluid production decrease as peak temperatures are reached. Large changes in the pattern of flow and influx of externally derived fluids may thus occur in metamorphic terranes when dehydration wanes or ceases and cooling begins. Inclusion of an impermeable horizon in the models further inhibits fluid circulation. Earlier, shallow hydrothermal models and interpretations based on the Rayleigh number may be inappropriate for characterizing fluid flow during prograde metamorphism at depth because they do not account for fluid production.     

Stable isotopic evidence for fluid infiltration during contact metamorphism in a multiply-metamorphosed terrane: the Reynolds Range, Arunta Block, central Australia

The Lander Rock Beds form the local basement of the Reynolds Range in the Arunta Inlier of central Australia. These dominantly quartzose and pelitic lithologies underwent low-grade ( c.   400  °C) regional metamorphism prior to contact metamorphism ( c.   2.5  kbar) around S-type megacrystic granitoids at 1820–1800  Ma. The Lander Rock Beds are overlain by metasediments of the Reynolds Range Group, which were subsequently intruded by granitoids at c. 1780  Ma. Regional metamorphism at 1590–1580  Ma produced grades varying from greenschist (400  °C at 4–5  kbar) to granulite (750–800  °C at 4–5  kbar) from north-west to south-east along the length of the Reynolds Range. Oxygen isotope ratios of the Lander Rock Beds were reset from 13.4±0.8 to as low as 6.7 adjacent to the contacts of the larger plutons, and to 10.3±1.1 around the smaller plutons. Biotite in all the major rock types found in the aureoles has δD values between −52 and −69, probably reflecting resetting by a cooling igneous+metamorphic fluid near the plutons. Sapphirine-bearing and other Mg- and Al-rich rock types have low δ¹⁸O values (4.0±0.7). The precursors to these rocks were probably low-temperature ( c. 200  °C) diagenetic–hydrothermal deposits of Mg-rich chlorite, analogous to those in Proterozoic stratiform precious metal and uranium deposits that form by the infiltration of basin brines or seawater. As in the overlying Reynolds Range Group, regional metamorphism involved little fluid–rock interaction and isotopic resetting.     

Contact metamorphic P–T–t paths from Sm–Nd garnet ages, phase equilibria modelling and thermobarometry: Garnet Ledge, south-eastern Alaska, USA

Sm–Nd garnet‐whole rock geochronology, phase equilibria, and thermobarometry results from Garnet Ledge, south‐eastern Alaska, provide the first precisely constrained P–T–t path for garnet zone contact metamorphism. Garnet cores from two crystals and associated whole rocks yield a four point isochron age for initial garnet growth of 89.9 ± 3.6 Ma. Garnet rims and matrix minerals from the same samples yield a five point isochron age for final garnet growth of 89 ± 1 Ma. Six size fractions of zircon from the adjacent pluton yield a concordant U–Pb age of 91.6 ± 0.5 Ma. The garnet core and rim, and zircon ages are compatible with single‐stage garnet growth during and/or after pluton emplacement. All garnet core–whole rock and garnet rim‐matrix data from the two samples constrain garnet growth duration to ≤5.5 my. A garnet mid‐point and the associated matrix from one of the two garnet crystals yield an age of 90.0 ± 1.0 Ma. This mid‐point result is logically younger than the 90.7 ± 5.6 Ma core–whole rock age and older than the 88.4 ± 2.5 Ma rim‐matrix age for this sample. A MnNaCaKFMASH phase diagram (P–T pseudosection) and the garnet core composition are used to predict that cores of garnet crystals grew at 610 ± 20 °C and 5 ± 1 kbar. This exceeds the temperature of the garnet‐in reaction by c. 50 °C and is compatible with overstepping of the garnet growth reaction during contact metamorphism. Intersection of three reactions involving garnet‐biotite‐sillimanite‐plagioclase‐quartz calculated by THERMOCALC in average P–T mode, and exchange thermobarometry were used to estimate peak metamorphic conditions of 678 ± 58 °C at 6.1 ± 0.9 kbar and 685 ± 50 °C at 6.3 ± 1 kbar, respectively. Integration of pressure, temperature, and age estimates yields a pressure‐temperature‐time path compatible with near isobaric garnet growth over an interval of c. 70 °C and c. 2.3 my.     

Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn–Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.     

Alkaline Intrusives at the East Foot of the Taihang-Da Hinggan Mountains: Chronology, Sr, Nd and Pb Isotopic Characteristics and Their Implications

Based on the Rb-Sr isochron dating results, this paper suggests that the alkaline intrusive belt at the east foot of the Taihang-Da Hinggan Mountains were formed between 135 and 122 Ma. And the alkaline intrusives in the north and south sections of this belt have entirely different Sr, Nd and Pb isotopic characteristics, i.e., all the rocks in the south section have positive εSr(t) and negative εNd(t) values and all those in the north have the opposite values. On the εSr(t) versus εNd(t) correlation diagram, the samples from the south are concentrated along the enriched mantle evolution trend lines and nearby, while those from the north fall along the depleted mantle trend lines and nearby. On the Pb isotope composition diagram, most of the samples from the south section fall on the mantle Pb evolution line and nearby, while those from the north lie between the Pb evolution lines of the mantle and the erogenic belt. The above-stated isotopic characteristics not only indicate that the source rocks of     

大别山北缘中生代火山—侵入岩锶—钕同位素组成特征及其物质来源

报道了大别山北缘中生代（早白垩世）不同成因系列火山－侵入岩的14个Rb-Sr,Sm-Nd同位素分析数据，其中正长岩类ISr值为0．70943－0．71014，εNd值为－14．7－15．8，亏损地幔两阶段模式年龄TDM为2．12－2．20Ga；高钾钙碱性系列岩石ISr值为0．70750－0．71054，εNd值为－17．2－－19．1，TDM为2．32－2．48Ga，橄榄安粗岩系列岩石ISr值为0．70873－0．70912，εNd值为－20．7－21．7，TDM为2．60－2．68Ga。通过与基底变质岩Sm-Nd同位素组成对比，认为高钾钙碱性系列主要由类似于大别群的地壳岩石衍生而成，正长岩类是由大别群和少量较年轻地壳岩石（卢镇关群和佛子岭群？）衍生而成的，而橄榄安粗岩系列则来源于富集地幔并可能混入少量更古老的类似于泰山群的地壳组分。     

A Sr isotope study on fluorite and siderite from post-orogenic mineral veins in the eastern Harz Mountains,Germany

Rb–Sr isotope data for siderite and fluorite from sediment-hosted epithermal mineral veins in the eastern Harz Mountains (Germany) are presented. Several fluorite and siderite-bearing paragenetic stages have been proposed for these veins, with the most important mineralization being related to a quartz–sulfide and a subsequent calcite–fluorite–quartz stage, which occurred at 226±1 and 209±2 Ma, respectively. Our Rb–Sr data do not permit the identification of distinct generations of siderite and fluorite, but rather reveal straight internal mixing relations, reflecting mixing of fluids or differential fluid–rock interaction processes. This indicates merely two significant phases of mineral deposition related to the quartz–sulfide and calcite–fluorite–quartz stages. It is shown that the Paleozoic sedimentary host rocks of the veins are the most likely source for the siderite Sr, whereas fluorite displays a two-component mixture between sedimentary Sr and radiogenic Sr derived from locally occurring Permian metavolcanic rocks. Editorial handling: B. Lehmann     

Domainal variations of U–Pb monazite ages and Rb–Sr whole-rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T >650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm³‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/D_eff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.     

Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island,Canada

This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ～50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO₂)^fluid pseudosections that suggest achievement of CO₂‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ¹³C and δ¹⁸O values of carbonate minerals. Although the up to 5‰ decrease of both δ¹³C and δ¹⁸O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ¹³C is attributed to fluid‐mediated exchange with organic low‐¹³C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ¹⁸O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high‐temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO₂)^fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling.     

Conditions of contact metamorphism, Papoose Flat Pluton, eastern California, USA: implications for cooling and strain histories

Abstract Petrological study of highly strained carbonate and pelitic rocks within the contact aureole surrounding the western part of the Papoose Flat pluton yields thermal profiles (plots of metamorphic temperature versus distance) across the aureole that show temperature gradients which are relatively flat and narrow (<100m). The gradients occur close to the contact and indicate a slight decrease in temperature from 500–550°C at the pluton/wall rock contact to 450–500°C at the outer margin of the aureole. One thermal profile across low-strain metasedimentary rocks located in the southern part of the aureole shows that thermal effects from emplacement extend no further than 600 m from the contact. Coexistence of andalusite and cordierite in pelitic rocks of the aureole constrain pressures to <4 kbar. Thermal modelling using an analytical solution of the conductive heat flow equation for a rectangular-shaped pluton reproduces the observed thermal maxima and profile shape. Conductive rather than convective cooling also is supported by isotopic and field evidence for limited fluid flow along the strongly deformed margin of the pluton. Simple thermal models coupled with observed high-temperature deformation features and a measured 90% attenuation of stratigraphic units in the plastically deformed western part of the pluton's aureole indicate that strain rates may have been of the order of 10^-12s^-1. Evidence for episodic heating, such as two distinct generations of andalusite growth in pelites from the aureole, alternatively may indicate a longer heating event and, therefore, slower strain rates. Thermal models also indicate that parts of the pluton still may have been above the solidus during deformation of the pluton margin and aureole.     

陕西金堆城钼矿区花岗岩Sr、Nd、Pb同位素特征及其地质意义

通过对陕西金堆城钼矿区花岗斑岩体和八里坡斑岩体进行地球化学测试,测得金堆城斑岩体的SiO2含量为72.89%～74.06%,MgO为0.07%～0.3%,稀土总量为43.29×10-6～93.94×10-6,稀土元素配分曲线呈右倾型,具有明显的铕负异常和弱的负铈异常(δEu为0.43～0.78,δCe为0.74～0.86),富集大离子亲石元素K、Rb、U、Th和Sr等元素,亏损Ba、P和Ti等元素。八里坡斑岩体的SiO2含量为69.87%～70.80%,Al2O3 14.93%～15.46%,MgO 0.28%～0.48%,Sr/Y比值大于60,稀土总量为125.23×10-6～139.63×10-6,铕为无异常或微弱的正异常(δEu为0.98～1.04),铈为微弱的负异常(δCe为0.92～0.96),富集Ba、U、K等大离子亲石元素,而亏损P、Ta和Ti等元素。金堆城斑岩体和八里坡斑岩体的岩石类型为I型花岗岩,Pb同位素显示金堆城斑岩体的Pb主要来自下地壳,但有地幔物质的加入,八里坡斑岩体的Pb主要来自下地壳。金堆城斑岩体的ε(Nd,t)值为较低负值(-13.8～-15.2),但ε(Sr,t)变化较大,为-46.4～13.6,八里坡斑岩体具有负低ε(Nd,t)值(-20.4)和正高ε(Sr,t)值(64.5～65.2)。金堆城花岗斑岩和八里坡花岗斑岩的Sr、Nd、Pb同位素与华北地块相似,这两个岩体的源区为华北地块组成部分。     

Elemental mobility in subduction metamorphism: insight from metamorphic rocks of the Franciscan Complex and the Feather River ultramafic belt,California

The degree of element mobility in subduction metamorphism has generated much debate; some workers advocate considerable mobility during metamorphism, whereas others postulate minimal mobility. We assess this issue by examination of major and trace element concentrations and Pb-, Nd-isotopic data for 39 mafic metavolcanic rocks from the Franciscan subduction complex, related units of coastal California, and the Feather River ultramafic belt of the northern Sierra Nevada, California; these samples span a wide range of metamorphic grade. We conclude that these rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios, with the exception of some mobile large ion lithophile elements such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of these metabasalts occurred in a largely closed system. Lack of light rare earth element enrichment in the rocks demonstrates lack of chemical exchange with subducted metasediments. Relatively low SiO₂ content (<48 wt.%) of many of the metamorphic rocks and the lack of correspondence between silica depletion and metamorphic grade suggests that the silica depletion resulted from seafloor hydrothermal alteration before subduction. In spite of demonstrated mobility of Pb, and possible mobility of Nd, isotopic ratios of Pb and Nd were not modified during subduction metamorphism. In contrast to our results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert and the overlying mantle. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba- and Pb-rich fluids interacted more intensely with metabasalt at the higher grades of metamorphism. Comparison of these results with studies of the active Mariana forearc suggests that fluids interacting with the mantle wedge up-dip of the region of magma genesis are derived from subducting sediments overlying the down-going plate.     

青海南部二叠纪火山岩Sr、Nd、Pb同位素特征及地质意义

对青海南部治多-杂多一带二叠纪火山岩进行了全岩Sr、Nd、Pb同位素分析, 样品的I_Sr=0.70333~0.70417, ε_Nd(t) =4.1~5.5, 206Pb/204Pb=17.547~17.986, 207Pb/204Pb=15.476~15.584, 208Pb/204Pb=37.778~37.976.结合岩石学、地球化学研究表明, 该套火山岩为OIB型, 形成于板内伸展拉张的裂谷环境, 属于青藏高原及邻区晚古生代裂谷系统, 原始岩浆可能来自亏损的古软流圈, 裂解机制与古地幔柱活动有关.样品铅同位素的V₁=31.39~46.78, V₂=22.38~42.45, 落入扬子铅同位素省区域, 表明早二叠世青海南部地区具亲扬子性, 揭示出昌都地块在晚古生代属于特提斯区, 为泛华夏陆块群或扬子板块的一个组成部分.      

新疆阿尔泰西北部白哈巴一冲乎尔一带加里东和海西两期区域变质作用的厘定

通过对新疆西北部阿尔泰白哈巴—冲乎尔一带详细的野外地质填图和综合研究,在原划分的奥陶纪、震旦—寒武纪地层中分别采获了早泥盆世双壳化石,获得了加里东期、海西期不同期次的同位素地质年龄,在浅变质岩中划分出了石英岩状结构亚带和刺状结构亚带,在区域动力热流变质岩中划分出蓝晶石带、矽线石带、铁铝榴石带、十字石带等。新的证据表明,研究区白哈巴地区发育加里东期和海西期两期区域低温动力变质,南东部发育两期区域动力热流变质,海流滩为加里东期,冲乎尔则为海西期,沿库根断裂带产生叠加。这一划分使争论了近半个世纪的该区变质类型和变质世代的问题有了新的答案。     

峨眉山玄武岩Sr、Nd、Pb同位素特征及其物源探讨

选择峨眉山玄武岩区2个出露最全的云南永胜大迪里剖面和宾川上仓剖面进行了Sr、Nd、Pb同位素地球化学研究.结果表明, 少数样品的Pb同位素与Hanan和Graham定义的C组分相似, 而大多数样品则不在C组分范围之内, 说明除地幔柱物质外, 有岩石圈物质的加入.在多元同位素图解上, 峨眉山玄武岩位于EMⅠ、EMⅡ和DMM三端元之间, 表明其源区可以由地幔柱、富集的岩石圈地幔和地壳不同程度的混合来解释.结合已有的微量元素资料分析, 其中的地壳组分主要为下地壳, 而早期玄武质岩浆在上升过程中由于通道不畅通, 有较多的上地壳组分的混染.岩石圈地幔的富集作用可能与地幔柱释放出的小体积富Na、P而贫K的流体交代作用有关.粗面岩的同位素组成和玄武岩接近, 说明粗面岩是玄武质岩浆分离结晶作用形成的.      

南极乔治王岛中——新生代岩浆岩Sr—Nd—Pb同位素组成及源区特征

对乔治王岛１１个火山岩和２个侵入岩样品做了Ｓｒ－Ｎｄ－Ｐｂ同位素分析，其中（８７Ｓｒ／８６Ｓｒ）ｉ＝０．７０３２６～０．７０３９２，εＮｄ＝３．０２～６．７２，２０６Ｐｂ／２０４Ｐｂ＝１７．７７６～１８．５１５，２０７Ｐｂ／２０４Ｐｂ＝１５．．５０６～１５．５７１，２０８Ｐｂ／２０４Ｐｂ＝３７．８６８～３８．３０８。根据以上同位素组成及其相互关系并结合岩石学和微量元素特征及火山岩中熔融包裹体成分，得出以下结论：（１）研究的乔治王岛岩浆岩起源于亏损地幔（ＤＭＭ）和地幔流体交代成因的富集地幔ＥＭＩ混合形成的源区；（２）该源区具有广义Ｄｕｐａｌ异常，它正是由于ＥＭＩ组份加入即通过流体交代作用而产生的；（３）流体主要是俯冲深海沉积物脱碳酸盐作用而释放出的富ＣＯ２流体。     

河北矾山钾质碱性超镁铁岩—正长岩杂岩体Sm—Nd年龄和Sr、Nd同位素特征

河北矾山侵入杂岩,由层状钾质超镁铁岩--正长岩组成,已知它含有大型磁铁矿磷灰石矿床.现在已测定了岩体的岩石和矿物的Sm-Nd同位素资料,并获得了杂岩体的Sm-Nd同位素年龄.4个矿物样品和2个全岩样品Sm-Nd等时线年龄为243.4±9.7Ma,INd=0.512045,εNd(t)=-5.4;4个矿物样品和7个全岩样品等时线年龄为239±19Ma,INd=0.512055,εNd(t)=-5.3.这些年龄资料表明,矾山岩体可能侵位于早三叠.而矾山岩体的Rb-Sr等时线年龄为218±8Ma,Isr=0.70554,εsr=18.4的事实可能暗示矾山岩体在中三叠才完全固结并达到Rb-Sr体系封闭的温度.上述矾山岩体的Sr、Nd同位素特征表明,矾山岩体的物质源自富集的上地幔.