Plagioclase porphyroblast growth and its relation to deformation in the Alhamilla unit (Sierra Alhamilla, Betic Cordilleras, SE Spain)

Abstract. Plagioclase porphyroblasts from silvergrey schists belonging to the Nevado Filabride Complex in the Sierra Alhamilla (Betic Zone, SE Spain) are interpreted as having been formed preand synkinematically with respect to the second phase of deformation. Different types of inclusion patterns represent 'snap-shots'(high growth-rate/strain-rate ratio features) of the formation of a diffentiated crenulation cleavage during this second phase of deformation, by the processes of kinking, crenulation and associated differentiation.
Regional considerations indicate an Alpine age for this tectono-metamorphic event, which can be explained by the'hot emplacement'of the higher Nevado Filabride units. The observed structural evolution is not consistent with a pre-Alpine polyphase deformation history.     

Low-grade regional metamorphism in the Mesozoic-Cenozoic volcanic sequences of the Central Andes

ABSTRACT All the Mesozoic and Cenozoic volcanic rocks of the Central Andes (from southern Ecuador to central Chile), except Recent ones, have been affected by episodes of regional metamorphism, without change in texture and structure. The metamorphism, which ranges from low zeolite to greenschist facies, can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities. Some sequences display metamorphic patterns transitional to ocean-floor and to geothermal field types, reflecting variations along and across the Andes in tectonic setting and thermal gradients. Volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the Central Andes. The episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series, where strata of higher grade may even overlie those of lower grade. The existence of permeability-controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity, rate of reaction and P_fluid were acting, in addition to temperature and P,_tot overall gradients, during the regional metamorphism. The alteration is accompanied by chemical changes and disturbances of the K-Ar and Rb-Sr isotope systems. Similarities between Mesozoic facies series in the western and eastern flanks of the Andes are consistent with a mechanism of ensialic spreading-subsidence.     

Solubility of Anhydrite, CaSO4, in NaCl-H2O Solutions at High Pressures and Temperatures: Applications to Fluid-Rock Interaction

Anhydrite solubility in H₂O–NaCl solutions was measuredat 6–14 kbar, 600–800°C and NaCl mole fractions(X_NaCl) of 0–0·3 in piston–cylinder apparatus.Solubilities were determined by weight changes of natural anhydritein perforated Pt envelopes confined with fluid in larger Ptcapsules. In initially pure H₂O at 10 kbar and 800°C, CaSO₄concentration is low (0·03 molal), though much largerthan at the same temperature and 1 kbar. Hematite-buffered experimentsshowed slightly lower solubilities than unbuffered runs. CaSO₄solubility increases enormously with NaCl activity: at 800°Cand 10 kbar and X_NaCl of 0·3, CaSO₄ molality is 200 timeshigher than with pure H₂O. Whereas CaSO₄ solubility in pureH₂O decreases with rising T at low T and P, the high-P resultsshow that anhydrite solubility increases with T at constantP at all X_NaCl investigated. The effects of salinity and temperatureare so great at 10 kbar that critical mixing between sulfate-richhydrosaline melts and aqueous salt solutions is probable at900°C at X_NaCl 0·3. Recent experimental evidencethat volatile-laden magmas crystallizing in the deep crust mayevolve concentrated salt solutions could, in light of the presentwork, have important implications regarding such diverse processesas Mount Pinatubo-type S-rich volcanism, high-f O₂ regionalmetamorphism, and emplacement of porphyry Cu–Mo ore bodies,where anhydrite–hematite alteration and fluid inclusionsreveal the action of very oxidized saline solutions rich insulfur. KEY WORDS: anhydrite; sulfur; solubility; metamorphic brines; granulites     

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Tourmaline in the Martinamor antiform occurs in tourmalinites(rocks with >15–20% tourmaline by volume), clasticmetasedimentary rocks of the Upper Proterozoic Monterrubio formation,quartz veins, pre-Variscan orthogneisses and Variscan graniticrocks. Petrographic observations, back-scattered electron (BSE)images, and microprobe data document a multistaged developmentof tourmaline. Overall, variations in the Mg/(Mg + Fe) ratiosdecrease from tourmalinites (0·36–0·75),through veins (0·38–0·66) to granitic rocks(0·23–0·46), whereas Al increases in thesame order from 5·84–6·65 to 6·22–6·88apfu. The incorporation of Al into tourmaline is consistentwith combinations of ^xAl(NaR)_–1 and AlO(R(OH))_–1exchange vectors, where ^x represents X-site vacancy and R is(Mg + Fe²⁺ + Mn). Variations in ^x/(^x + Na) ratios are similarin all the types of tourmaline occurrences, from 0·10to 0·53, with low Ca-contents (mostly <0·10apfu). Based on field and textural criteria, two groups of tourmaline-richrocks are distinguished: (1) pre-Variscan tourmalinites (probablyCadomian), affected by both deformation and regional metamorphismduring the Variscan orogeny; (2) tourmalinites related to thesynkinematic granitic complex of Martinamor. Textural and geochemicaldata are consistent with a psammopelitic parentage for the protolithof the tourmalinites. Boron isotope analyses of tourmaline havea total range of ¹¹B values from –15·6 to 6·8;the lowest corresponding to granitic tourmalines (–15·6to –11·7) and the highest to veins (1·9to 6·8). Tourmalines from tourmalinites have intermediate¹¹B values of –8·0 to +2·0. The observedvariations in ¹¹B support an important crustal recycling ofboron in the Martinamor area, in which pre-Variscan tourmaliniteswere remobilized by a combination of mechanical and chemicalprocesses during Variscan deformation, metamorphism and anatexis,leading to the formation of multiple tourmaline-bearing veinsand a new stage of boron metasomatism. KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes     

A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

Progress () of the infiltration-driven reaction, 4olivine +5CO₂ + H₂O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m³ volume of rock. Mineral and stableisotope compositions record that X_CO2, ¹⁸O_fluid, and ¹³C_fluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress     

Pressure-Temperature Evolution of a Late Palaeozoic Paired Metamorphic Belt in North-Central Chile (34{degrees}-35{degrees}30'S)

In the Chilean Coastal Cordillera, two units, the Western andEastern Series, constitute coeval parts of a Late Palaeozoicpaired metamorphic belt dominated by siliciclastic metasediments.The Western Series also contains rocks from the upper oceaniccrust and represents an accretionary prism. Omnipresent high-pressureconditions are reflected by Na–Ca-amphibole and phengitein greenschists. Peak PT conditions of 7·0–9·3kbar, 380–420°C point to a metamorphic gradient of11–16°C/km. Three unique occurrences of blueschistyield deviating conditions of 9·5–10·7 kbar,350–385°C and are interpreted as relics from the lowermostpart of the basal accretion zone preserving the original gradientof 9–11°C/km along the subducting slab. Pervasiveductile deformation related to basal accretion occurred nearpeak PT conditions. Deformation and PT evolution of the metapsammopeliticrocks is similar to that of the metabasites. However, a raregarnet mica-schist yields peak PT conditions of 9·6–14·7kbar, 390–440°C reflecting a retrograde stage aftercooling from a high-temperature garnet-forming stage. It isconsidered to be an exhumed relic from the earliest siliciclasticrocks subducted below a still hot mantle wedge. A retrogradeoverprint of all rock types occurred at 300–380°C.Continuous reactions caused crystal growth and recrystallizationwith abundant free water mostly under strain-free conditions.They record a pressure release of 3–4 kbar without erasingpeak metamorphic mineral compositions. The Eastern Series lacksmetabasite intercalations and represents a less deformed retro-wedgearea. In the study area it was entirely overprinted at a uniformdepth at 3 ± 0·5 kbar with temperatures progressivelyrising from 400°C to 720°C towards the coeval Late Palaeozoicmagmatic arc batholith. The interrelated pattern of PT datapermits a conceptual reconstruction of the fossil convergentmargin suggesting a flat subduction angle of 25° with continuousbasal accretion at a depth of 25–40 km and a short mainintrusion pulse in the magmatic arc. The latter was accompaniedby the formation of a thermal dome in the retro-wedge area,which remained stable relative to the vertically growing accretionaryprism characterized by cyclic mass flow. KEY WORDS: paired metamorphic belt; greenschist; blueschist; central Chile; thermobarometry     

Nature of the Precambrian metamorphic blocks in the eastern segment of Central Tianshan: Constraint from geochronology and Nd isotopic geochemistry

The Central Tianshan Tectonic Zone (CTTZ) is anarrow domain between an early Paleozoic southernTianshan passive continental margin and a late Paleo-zoic northern Tianshan arc zone, which is character-ized by the presence of numerous Precambrian meta-morphic basement blocks. Proterozoic granitoidgneisses and metamorphic sedimentary rocks,namely Xingxingxia and Kawabulag and Tianhugroups, are the most important lithological assem-blages in these metamorphic basement blocks, and alittle of …     

近20年我国气象灾害损失的初步分析

利用1989—2008年的全国自然灾害灾情资料,采用综合集成评价方法计算出气象灾害损失的综合指数,运用聚类分析法对近20年的气象灾害损失进行定级并分析其随时间的变化。结果显示,20年中前10年气象灾害的损失比较大,后10年损失显著减小;气象灾害导致的人口死亡数在近20年中呈明显的下降趋势;1991、1994、1996、1998年为气象灾害的重灾年。     

四川省九龙县黑牛洞富铜矿矿床黑云母40Ar-39Ar测年及其地质意义

四川省九龙县黑牛洞铜矿床为近期新发现的一个规模在中型以上,产于江浪变质核杂岩中的富铜矿床。本文测试了黑牛洞铜矿床中与成矿密切相关的黑云母的40Ar/39Ar年龄为136.43±0.77 Ma,说明黑牛洞矿床形成于燕山中期。结合矿区附近李伍富铜床矿床地质特征及成矿年龄,表明黑牛洞铜矿床与李伍铜矿床为同一时期形成的矿床,两者成矿与松潘－甘孜造山带大规模岩浆热事件有关,这对于重新认识李伍铜矿床外围找矿潜力和进一步指导找矿工作具有重要意义。     

The Ordovician Sierras Pampeanas–Puna basin connection: Basement thinning and basin formation in the Proto-Andean back-arc

The Ordovician Sierras Pampeanas, located in a continental back-arc position at the Proto-Andean margin of southwest Gondwana, experienced substantial mantle heat transfer during the Ordovician Famatina orogeny, converting Neoproterozoic and Early Cambrian metasediments to migmatites and granites. The high-grade metamorphic basement underwent intense extensional shearing during the Early and Middle Ordovician. Contemporaneously, up to 7000 m marine sediments were deposited in extensional back-arc basins covering the pre-Ordovician basement. Extensional Ordovician tectonics were more effective in mid- and lower crustal migmatites than in higher levels of the crust. At a depth of about 13 km the separating boundary between low-strain solid upper and high-strain lower migmatitic crust evolved to an intra-crustal detachment. The detachment zone varies in thickness but does not exceed about 500 m. The formation of anatectic melt at the metamorphic peak, and the resulting drop in shear strength, initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies P–T conditions. P–T–d–t data in combination with field evidence suggest significant (ca. 52%) crustal thinning below the detachment corresponding to a thinning factor of 2.1. Ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25% and 44%, constituting total crustal thinning factors of 1.7–2.0. While the migmatites experienced retrograde decompression during the Ordovician, rocks along and above the detachment show isobaric cooling. This suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface. Upper crustal extension and thinning is compensated by newly deposited sediments, maintaining constant pressure at detachment level. Thinning of the migmatitic lower crust is compensated by elevation of the crust–mantle boundary. The degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension, basin formation, and sediment thickness in the back-arc basin. The initiation of crustal extension in the back-arc, however, crucially depends on the presence of anatectic melt in the middle and lower crust. Consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension.