Amphibolite Rocks Research Articles

Orientation-related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the Diancang Shan, SW Yunnan, China

Sheared amphibolite rocks from Diancang Shan high-grade metamorphic complex along the Ailao Shan-Red River shear zone, southwestern Yunnan, China, show typical mylonitic microstructures. The mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole- and quartzofeldspathic domains. Microstructural analysis and P/T estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies. In the mylonitic amphibolites, there are two types of amphibole porphyroclasts, i.e. type I “hard” and type II “soft” porphyroclasts. They have their [001] crystallographic orientations subnormal and sub-parallel to the stretching lineation of the rocks, respectively. The two types of porphyroclasts show distinct deformation microstructures and sub-microstructures formed by various deformation mechanisms, which contribute in different ways to the generation of the fine-grained matrix. Shape preferred orientation analysis, misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type II porphyroclasts. The type I “hard” porphyroclasts are deformed mainly by mechanical rotation, work hardening and intragranular microfracturing. In contrast, the deformation of the type II “soft” porphyroclasts is mainly attributed to crystalline plasticity, i.e. twinning, dislocation creep and dynamic recrystallization. During the deformation of the type II porphyroclasts, the (100) [001] slip system plays a dominant role during deformation and grain size reduction of amphibole. Twinning along the active (100) slip system, in combination with dislocation creep (gliding and climbing) governs the nucleation of subgrains and formation of dynamically recrystallized fine grains, a process here named Twinning Nucleation Recrystallization.

Carboniferous high-potassium I-type granitoid magmatism in the Eastern Pontides: The Gümüşhane pluton (NE Turkey)

The Gümüşhane pluton, a high-K calc-alkaline I-type granodiorite/granite complex, forms an important component of the pre-Liassic basement of the Eastern Pontides (NE Turkey). In its eastern part, the pluton shows a compositional zonation ranging from biotite–hornblende granodiorite in the NW through biotite–hornblende granite to leucogranite/granophyre in the SE. Numerous mafic microgranular enclaves (up to ∼ 40 cm in diameter) suggest the former presence of globules of mafic melt during crystallization. Emplacement of the pluton occurred during the latest Early Carboniferous, as shown by the 320 ± 4 Ma 40Ar– 39Ar biotite/hornblende and 324 ± 6 Ma LA-ICP-MS U–Pb zircon ages. In Harker diagrams, samples of the different rock types exhibit well-defined data trends. With increasing SiO 2, the abundances of TiO 2, Al 2O 3, Fe 2O 3 tot, MnO, MgO, CaO, P 2O 5 and Sc decrease, but those of K 2O and Rb increase. However, the variations of Sr, Ba, (La/Yb) cn, Sr/Y and ∑ REEs vs. SiO 2 form distinctive groupings, which cannot be explained by a simple fractional crystallization. Chondrite-normalized (cn) REE patterns of granodiorite/granite samples show concave-upward shapes with (La/Yb) cn ranging from 5.2 to 12.4 and Eu/Eu* from 0.84 to 0.47, while there is almost no fractionation of the middle REE relative to the heavy REE. In primitive mantle-normalized element concentration diagrams, all rocks display marked negative anomalies in Ba, Nb/Ta, Sr, P and Ti, but positive anomalies in K and Pb. These geochemical features imply a fractionating mineral assemblage of clinopyroxene, amphibole and plagioclase without significant involvement of garnet. The granophyres are, on the other hand, characterized by higher K 2O/Na 2O and Rb/Sr ratios, lower (La/Yb) cn ratios (1.3 to 4.8) and more pronounced negative anomalies in Ba, Nb/Ta, Sr, Eu, P and Ti. Initial ε Nd values range from − 3.78 to − 5.30 and Nd model ages from 1.38 to 163 Ga. The magmas of the granite/granodiorite portion were probably generated by partial melting of high-potassic amphibolitic rocks, and those of the granophyres by a relatively felsic micaceous crustal source. The Gümüşhane pluton was emplaced at the wake of the low-pressure–high-temperature metamorphism, and is regarded as a late phase of Hercynian orogeny in the Eastern Pontides.

Rubies and Sapphires from Winza, Central Tanzania

Since late 2007, rubies and sapphires have been mined by hand methods from both eluvial and primary deposits at Winza in central Tanzania. The gem corundum is related to dikes of amphibolitic rocks that belong to the Paleoproterozoic Usagaran Belt. Based on crystal morphology, Winza corundum is subdivided into two types: prismatic-tabular-rhombohedral and dipyramidal. In general, medium red and dark (orangy) red top-quality rubies are rhombohedral. Pinkish red and purplish red rubies, as well as pink, purple, and blue (often strongly color zoned) sapphires are, for the most part, dipyramidal. The top-quality rubies are characterized by a distinct assemblage of long tube-, fiber-, needle-, or hair-like inclusions containing an orange-brown material (most likely limonite). The lower-quality material generally contains a larger amount of solid inclusions (mostly amphibole crystals), fissures, and growth features. Unique to corundum from this locality are bluish violet color zones oriented parallel to the prism and basal pinacoid, and occasionally also parallel to rhombohedral and dipyramidal faces. The relatively high Fe content of Winza rubies separates them from most other natural and almost all synthetic counterparts.

Gold-hosting supracrustal rocks on Storø, southern West Greenland: lithologies and geological environment

A gold prospect on central Storø in the Nuuk region of southern West Greenland is hosted by a sequence of intensely deformed, amphibolite facies supracrustal rocks of late Mesoto Neoarchaean age. The prospect is at present being explored by the Greenlandic mining company NunaMinerals A/S. Amphibolites likely to be derived from basaltic volcanic rocks dominate, and ultrabasic to intermediate rocks are also interpreted to be derived from volcanic rocks. The sequence also contains metasedimentary rocks including quartzites and cordierite-, sillimanite-, garnet- and biotite-bearing aluminous gneisses. The metasediments contain detrital zircon from different sources indicating a maximum age of the mineralisation of c. 2.8 Ga. The original deposition of the various rock types is believed to have taken place in a back-arc setting. Gold is mainly hosted in garnet- and biotite-rich zones in amphibolites often associated with quartz veins. Gold has been found within garnets indicating that the mineralisation is pre-metamorphic, which points to a minimum age of the mineralisation of c. 2.6 Ga. The geochemistry of the goldbearing zones indicates that the initial gold mineralisation is tied to fluid-induced sericitisation of a basic volcanic protolith. The hosting rocks and the mineralisation are affected by several generations of folding.

Fluid chemistry of the Palaeoproterozoic Fäboliden hypozonal orogenic gold deposit, northern Sweden: Evidence from fluid inclusions

A new ore province, the Gold Line, southwest of the Skellefte District, northern Sweden, is currently under exploration. The largest known deposit in the Gold Line is the hypozonal Fäboliden orogenic gold deposit. The mineralization is hosted by arsenopyrite-bearing quartz veins, within a steep shear zone in amphibolite facies metagreywacke host rocks. Gold occur in fractures and as intergrowths in arsenopyrite-löllingite, and as free grains in the silicate matrix of the host rock. The hydrothermal mineral assemblage in the proximal alteration zone is diopside, calcic amphibole, biotite, and minor andalusite and tourmaline. Primary fluid inclusions in the Fäboliden quartz veins show a CO2-CH4 or a H2S (±CH4) composition (the latter recognized for the first time in a Swedish ore deposit). The primary fluid inclusions are associated with arsenopyrite-löllingite (+gold) and the CO2-CH4 fluid was also involved in precipitation of graphite. A prevalence of carbonic over aqueous fluid inclusions is characteristic for a number of hypozonal high-temperature orogenic gold deposits. The Fäboliden deposit, thus, shows fluid compositions similar to other hypozonal orogenic gold deposits. The proposed main mechanism for precipitation of gold from the fluids, is a mixing between H2S-rich and H2O?-CO2±CH4 fluids. Fluid inclusion data indicate arsenopyrite-löllingite and graphite deposition at a pressure condition of about 4 kbar. Graphite thermometry indicates maximum temperatures of 520-560°C for the hydrothermal alteration at Fäboliden, suggesting that at least the late stages of the mineralizing event took place shortly after peak-metamorphism in the area, i.e. at c. 1.80 Ga.

Paleoproterozoic supercontinent: Origin and evolution of accretionary and collisional orogens exemplified in Northern cratons

The evolution of the North American, East European, and Siberian cratons is considered. The Paleoproterozoic juvenile associations concentrate largely within mobile belts of two types: (1) volcanic-sedimentary and volcanic-plutonic belts composed of low-grade metamorphic rocks of greenschist to low-temperature amphibolite facies and (2) granulite-gneiss belts with a predominance of high-grade metamorphic rocks of high-temperature amphibolite to ultrahigh-temperature granulite facies. The first kind of mobile belt includes paleosutures made up of not only oceanic and island-arc rock associations formed in the process of evolution of relatively short-lived oceans of the Red Sea type but also peripheral accretionary orogens consisting of oceanic, island-arc, and backarc terranes accreted to continental margins. The formation of the second kind of mobile belt was related to the activity of plumes expressed in vigorous heating of the continental crust; intraplate magmatism; formation of rift depressions filled with sediments, juvenile lavas, and deposits of pyroclastic flows; and metamorphism of lower and middle crustal complexes under conditions of granulite and high-temperature amphibolite facies that, in addition, spreads over the fill of rift depressions. The evolution of mobile belts pertaining to both types ended with thrusting in a collisional setting. Five periods are recognized in Paleoproterozoic history: (1) origin and development of a superplume in the mantle that underlay the Neoarchean supercontinent; this process resulted in separation and displacement of the Fennoscandian fragment of the supercontinent (2.51–2.44 Ga); (2) a period of relatively quiet intraplate evolution complicated by locally developed plume-and plate-tectonic processes (2.44–2.0 (2.11) Ga); (3) the origin of a new superplume in the subcontinental mantle (2.0–1.95 Ga); (4) the complex combination of intense global plume-and plate-tectonic processes that led to the partial breakup of the supercontinent, its subsequent renascence and the accompanying formation of collisional orogens in the inner domains of the renewed Paleoproterozoic supercontinent, and the emergence of accretionary orogens along some of its margins (1.95–1.75 (1.71) Ga); and (5) postorogenic and anorogenic magmatism and metamorphism (<1.75 Ga).

PROTOLITH COMPOSITION OF CORDIERITE-GEDRITE BASEMENT ROCKS AND GARNET AMPHIBOLITE OF THE BEARPAW LAKE AREA OF THE THOR-ODIN DOME, MONASHEE COMPLEX, BRITISH COLUMBIA, CANADA

The Thor–Odin dome of the Monashee complex, southeastern Canadian Cordillera, British Columbia, contains basement gneiss of North American tectonic affinity, including a distinctive cordierite–gedrite rock that is restricted in occurrence to the basement rocks and a regionally occurring garnet amphibolite. All of these rocks were penetratively deformed and metamorphosed at upper-amphibolite to lower-granulite facies conditions in the Cordilleran orogeny during the Late Cretaceous to Paleocene. The cordierite–orthoamphibole (dominantly gedrite) unit appears to define a discontinuous marker horizon or horizons within the Paleoproterozoic basement paragneiss. The cordierite–gedrite rocks have a unique bulk-rock composition that is characterized by depletions in the alkali elements and Ca, and enrichments in Al, Mg and Fe. They have flat rare-earth element (REE) patterns, depletions in most low field-strength elements (LFSE), and enrichments in high field-strength elements (HFSE). The measured 87Sr/86Sr values for these rocks vary from 0.74923 to 0.85962, and the eNd(today) values vary from −15.3 to −20.6. The cordierite–gedrite rocks are interpreted as Paleoproterozoic mafic volcanic rocks that were hydrothermally altered, likely prior to metamorphism. Hydrothermal alteration explains their distinctive bulk-rock composition. Interlayered and spatially associated lenses of garnet amphibolite are of uncertain age, although they are likely Precambrian, on the basis of their Nd isotopic composition. The garnet amphibolite rocks are characterized by high Mg and Ti values and depletions in the alkali elements. They have flat REE patterns, depletions in most LFSE and enrichments in HFSE. The garnet amphibolite samples have measured 87Sr/86Sr values that vary from 0.70953 to 0.74319, and eNd(today) values that vary from −0.8 to −7.3. The garnet amphibolite rocks are interpreted as Proterozoic metamorphosed mafic dykes or possibly volcanic rocks that postdated the formation of the protolith of the cordierite–gedrite rocks.

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D1, D3, D4, and D5, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D1 and syn- to post-D3. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D1 veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D1) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D3 and D4 phases. Little is known about the nature of the D1 ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D1 indicates a high temperature for the fluid. The D3 hydrothermal phase coincided with peak metamorphism. D4 fluids were hypersaline, high temperature, CO2-poor, and H2S-poor.

Indications of a major Neolithic trade route? An archaeometric geochemical and Sr, Pb isotope study on amphibolitic raw material from present day Europe

In order to interpret pre-historic cultural interactions, the provenance of amphibolitic raw material used for flat-axes and adzes in lower to middle Neolithic cultures throughout present day Germany were investigated by elemental and Sr, Pb isotopic methods. Within all settlements studied, a homogeneous actinolite-hornblende schist rock type (AHS) was found to be massively dominating, with a distinct petrography of needle-shaped actinolite interwoven with single larger grains of hornblende along with calcic plagioclase and large amounts of ilmenite. Geochemically, the AHS group is very homogeneous and has a signature of an enriched basaltic precursor with high concentrations of particularly the LIL-elements. The geochemical signature is relatively rare and can not be matched in nearby geological outcrops, wherefore the conclusion of “imported material” is quickly reached. Strontium and Pb isotopic analyses of the AHS were compared to the isotopic composition of amphibolitic rocks with similar petrography and trace elemental signatures within possible archaeological trade regions. The isotopic data of the archaeological material point roughly to a Proterozoic age of the stone used; an age which can be reasonable matched to a single outcrop situated at Jistebsko within the Czech Republic. This area further shows archaeological traces of prehistoric mining. Based on petrographic, geochemical and isotopic evidence, this area is here presented as the provenance area of the stone raw material, which later spread throughout prehistoric Europe – establishing contact and trade routes between Neolithic cultures.

Structural and geochemical studies on amphibolite and greenschist-facies rocks in the Kulidjik river valley, eastern Rhodope, Bulgaria: preliminary results

Structural and geochemical studies on amphibolite and greenschist-facies rocks in the Kulidjik river valley, eastern Rhodope, Bulgaria: preliminary results

Nature and significance of Late Cretaceous ophiolitic rocks and their relation to the Baskil granitic intrusions of the Elazığ region, SE Turkey

Abstract The Elazığ region in SE Turkey comprises, in descending order, the Palaeozoic-Mesozoic Malatya-Keban platform, an ensimatic island arc unit (i.e. Elazığ magmatic rocks-Yüksekova complex), and ophiolitic rocks (i.e. Kömürhan) of Late Cretaceous age. All of these were intruded by the Baskil granitic rocks. These tectonomagmatic-stratigraphic assemblages were emplaced over the Middle Eocene volcano-sedimentary Maden complex to the south during the evolution of the SE Anatolian orogen. The Kömürhan ophiolite exhibits an intact ophiolite pseudostratigraphy. The base of this has been metamorphosed to amphibolite facies during intraoceanic subduction-thrusting. The amphibolitic rocks were intruded by synkinematic granitic rocks (Baskil magmatic rocks). The ensimatic island arc volcanic rocks are widely distributed in the region. The contact of the volcano-sedimentary unit with the underlying Kömürhan ophiolite is a thrust dipping to the north. The rock assemblages of the volcano-sedimentary unit suggest formation of small volcanic edifices above a subduction zone, coupled with debris-flow deposits and volcaniclastic turbidites. The whole-rock and mineral chemistry of the Kömürhan ophiolite and the ensimatic island arc volcanic rocks suggests that they represent a comagmatic tholeiitic suite, formed in the Late Cretaceous in a suprasubduction zone (SSZ) setting. The amphibolites beneath the Kömürhan ophiolite indicate derivation from an island arc tholeiite (IAT) protolith. The geological and geochemical evidence from the Elazığ region suggests the following evolutionary scenario. The Kömürhan ophiolite was formed above a north-dipping subduction zone between the Arabian platform to the south and the Tauride platform to the north in Late Cretaceous ( c. 90 Ma). An ensimatic island arc assemblage was then built on the SSZ-type crust. The metamorphic sole was formed by metamoprhism of IAT-type basalts that were detached from the front of the overriding Kömürhan ophiolite and then underplated. These units were then accreted to the base of the Tauride active margin to the north, where both units were cut by the Baskil granitic rocks around 85 Ma.

U-Pb ages (3.8–2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, Superior Craton, Canada

We report new U-Pb ages and Sm-Nd isotope data for an Eoarchean volcano-sedimentary sequence in the northeast Superior Province of northern Quebec, Canada. The Nuvvuagittuq supracrustal sequence is characterized by mafi c amphibolite rocks with rare felsic schists, accompanied by ultramafi c sills, oxide-rich and quartz-rich iron formations, and possible conglomeratic units, with peak metamorphism having reached upper amphibolite facies. The sequence crops out as an open, shallow, south-plunging synform with tonalites in the center and around the exterior margins of the fold structure. A 3817 ± 16 Ma age from a felsic schist is interpreted to represent a maximum age for the sequence, whereas a 3661 ± 4 Ma age from the enclosing tonalites suggests a minimum age. Detrital zircons from a conglomeratic unit yielded similar old ages of ca. 3.78 Ga. Younger ages of ca. 3.66 and 3.3 Ga from a conglomeratic unit and a felsic schist, and 2.7 Ga ages from an amphibolite gabbro and a pegmatite dike, underline the polymetamorphic history of the sequence. Initial Nd isotope compositions (eNd 3.8 = −0.5 to + 2.3) for mafi c amphibolites and felsic schists are indicative of a moderately depleted mantle. Sm-Nd analyses of a 2.7 Ga pegmatite dike and the ca. 3.66 Ga tonalites both yield Nd depleted mantle model ages (TDM) of ca. 3.9 Ga, indicating that they were derived from a crust of juvenile Eoarchean origin. We propose that fragments of Paleoarchean to Eoarchean terranes that rim the Superior Province represent remnants of a larger terrane(s) that was rifted and dismembered prior to the formation of the 3.0‐2.7 Ga Superior Craton.

Characteristics of ophiolite-related metamorphic rocks in the Beysehir ophiolitic mélange (Central Taurides, Turkey), deduced from whole rock and mineral chemistry

Small outcrops of the metamorphic rocks of the Beysehir ophiolite appear to the west of Gencek and to the south of Durak (South of Beysehir Lake) in the Central Tauride Belt in Turkey. Amphibolitic rocks in the ophiolitic mélange have an igneous origin. Protoliths of these rocks were probably alkali basalts, gabbros or some ultramafic cumulates, such as pyroxenite. The amphibolites of the Beysehir Ophiolite can be divided into four groups: (1) amphibole+garnet+plagioclase±epidote (as secondary minerals)±opaque such as ilmenite±accessory minerals such as sphene and apatite; (2) amphibole+pyroxene+plagioclase±epidote±accessory minerals such as sphene, apatite±chlorite, calcite (as secondary mineral); (3) amphibole±plagioclase±opaque±accessory minerals; (4) amphibole+plagioclase±epidote±biotite and muscovite±opaque±accessory minerals. These metamorphic rocks show mainly granoblastic, grano-nematoblastic, porphyroblastic and/or poikiloblastic textures. All amphiboles in the amphibolites are calcic and cluster in the range from magnesio-hastingsite, pargasite to actinolite. Amphibole compositions are characterized by SiO 2=(38.02–54.3%), Al 2O 3=(1.5–12.8), FeO=(10.03–14.67%), K 2O=(0.2–1.8%), MgO=(5.5–15.7), Mg*=(0.3–0.8). The amphibolites show an alkaline to subalkaline character. However, the primitive mantle normalized incompatible trace element diagram shows close similarity with the typical ocean island basalt (OIB) pattern. The Rock/Chondrite normalized REE diagram of the amphibolites also confirms their OIB signature. Tectonomagmatic discrimination diagrams based on the immobile trace elements suggest a mostly within-plate alkali basalt (WPB) environment. Beysehir ophiolitic mélange contains amphibolites from ophiolite-related metamorphic rocks, but the matrix of the Beysehir ophiolitic mélange is not metamorphosed. Blocks of metamorphic rocks and the ophiolitic rocks may have been incorporated into the ophiolitic mélange in an oceanic environment during the Late Cretaceous by tectonic forces.

Geochemical heterogeneity and element mobility in deeply subducted oceanic crust; insights from high-pressure mafic rocks from New Caledonia

Bulk-rock major and trace element geochemistry of a range of eclogite, garnet blueschist and garnet amphibolite rocks from northern New Caledonia has been determined in order to geochemically characterise subducted oceanic crust. The rocks experienced peak metamorphic conditions of 1.9 GPa and 600 °C and represent excellent samples of oceanic crust that was subducted to depths of approximately 60 km. The rocks can be divided into seven rock types that respectively have geochemical characteristics of enriched and normal mid-ocean ridge basalt, back-arc basin basalt, alkaline basalt, plagioclase-rich cumulate, seafloor-altered basalt and Fe–Ti basalt. All of the samples studied represent a single slice of oceanic crust interpreted to have formed in a back-arc or marginal basin setting. Examination of modern oceanic crust suggests that most subducting crust also contains a diverse range of mafic rock-types. The presence of minor amounts of alkaline and seafloor-altered basalts in the slab can greatly influence the recycling of incompatible elements and the depth of fluid release during subduction. Comparison of the high-grade metamorphic rocks with equivalent igneous rocks from western New Caledonia demonstrates that the main chemical variations of the rocks are related to differences in their magmatic history and to different degrees of seafloor alteration, whereas high-pressure metamorphism produced only minor changes. There is evidence for some depletion of LILE and B during subduction in a few of the analysed samples. However, most of the blueschists and eclogites with protoliths showing high LILE contents such as the back-arc basin and alkaline basalts still contain high LILE contents of 10–100 times the amount found in normal mid ocean ridge basalts. Therefore, even fluid mobile elements (B, LILE) may be efficiently subducted to sub-arc depths. Trace elements are most likely to be removed from the slab in regions of elevated temperature or in zones of intense fluid–rock interaction or partial melting. In contrast to the trace elements, large volumes of fluid are liberated from mafic rocks prior to eclogite-facies metamorphism, providing evidence for a decoupling of fluid and trace element release in subducted oceanic crust.

Timing of high‐grade metamorphism: Early Palaeozoic U–Pb formation ages of titanite indicate long‐standing high‐T conditions at the western margin of Gondwana (Argentina, 26–29°S)

Abstract Concordant U–Pb ages of c. 530–510 Ma and c. 470–420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high‐T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26–29°S). The U–Pb ages document the time of titanite formation at high‐T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were &gt; c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U–Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U–Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U–Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation‐enhanced recrystallization of titanite and/or different thermal peaks during a long‐standing, geographically fixed, high‐T regime in the mid‐crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region.

Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (∼15 km 2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An 7–20) and microcline (Or 92–98), with lesser biotite (Fe ∗=0.8–0.9) and accessory muscovite (Fe ∗=0.7–0.8). U–Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/ 206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb–Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U–Pb age. There is evidence for open-system behaviour in the Rb–Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm–Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids ( I Sr=0.7110±14; ε Nd=−3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An 45–68) and magnesio-hornblende (Fe ∗=0.3–0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios ( I Sr=0.7058; ε Nd=+2.9) that approach model depleted mantle at 968 Ma. The metagabbro is chemically and isotopically similar to mafic metavolcanic and related rocks that have been considered to represent ocean-floor and island-arc basaltic magmas. All available data are compatible with the idea that these rocks represent the products of convergent margin processes during the Early Neoproterozoic, a conclusion that could have important implications for the construction of the Rodinia supercontinent.

RHIZOSPHERE CHEMISTRY IN ACID FOREST SOILS THAT DIFFER IN THEIR DEGREE OF AL-SATURATION OF ORGANIC MATTER

The objective of this research was to study the rhizosphere of tree species of acid forest soils derived from amphibolite (AMP) and granite (GR) rocks, so that the influence of the type of pedogenesis (generated by these materials) on its chemistry could be established. Umbric A 1 horizons were sampled in three AMP soils (Lithic Hapludands, Andic Dystrudept), under oak, pine, and eucalyptus stands, and in three nearby GR soils (Typic Dystrudepts) under identical stands. The A 1 GR horizons had a lower pH (4.2 vs 4.6), and greater aqueous soluble A 1 (0.40 vs 0.06 mmol kg -1 ) and NH 4 Cl-extractable Al contents (6.8 vs 2.6 cmol c kg -1 ) than the A 1 AMP horizons. Mean values of sodium pyrophosphate extractable-Al (RI P ) and ammonium oxalate extractable-Al (Al o ) in the A 1 AMP horizons were 12.0 and 16.2 g kg -1 , respectively, and the molar ratio between Al p and sodium pyrophosphate extractable-C (C ) ranged between 0.1 and 0.2. Mean values of Al p , Al o , and Al p / C p in the A 1 GR horizons were significantly (P < 0.05) smaller (5.2 g kg -1 , 5.3 g kg -1 , and 0.06, respectively). In the A 1 GR horizons, we observed a lower pH (significant at P < 0.05) in the rhizosphere than in the bulk soils, a pattern which was not observed in the A 1 AMP horizons. The balance of nutritive cations and anions between rhizosphere and bulk soils did not explain the patterns of pH change. Based on the relationships found among organic acids, soil pH, and Al p /C p , we propose that, in the At GR horizons, where organic matter is Al-undersaturated, the increased organic acidity produced in the rhizosphere cannot be completely buffered by Al alkalinity, whereas in the AMP soils, where mineral amorphous Al coexists with Al-humus complexes, organic compounds become stabilized with reactive Al surfaces, maintaining organic acidity at low levels, even in the rhizosphere (whenever the stabilization capacity of these surfaces is not exceeded). Our results imply that both variations in organic matter content and the degree of metal saturation of the organic matter need to be considered when investigating the causes behind changes in soil pH in the rhizosphere of acid forest soils.

Caracterização isotópica de metabasitos e anfibolitos dos Grupos Açungui e Setuva na Porção Sul da Faixa Ribeira

Pb, Rb, Sr, Sm and Nd isotope analyses were carried out on amphibolitic rocks from Campo Largo (PR), Rio Branco do Sul (PR) and Adrianópolis (PR) and for the metabasite from Adrianópolis and Apiaí (SP), all belonging to the Açungui and Setuva Groups, southern portion of the Ribeira Belt. These occurrences were chosen because each exhibits geochemical data indicative of a mantle source, having different signatures. Geochronologic determinations are available only for the Apiaí metagabbro with Neoproterozoic ages of 617 ± 4 Ma (U/Pb in zircon) and 839 ± 85 Ma (Rb/Sr, whole rock). Age determinations by Sm/Nd mineral isochron using whole rock, plagioclase and pyroxene yield values of 885 ± 53 Ma for the Apiaí metagabbro and 820 ± 84 Ma for the Adrianópolis metabasite. These results agree within error with the Rb/Sr isochron of 839 ± 85 Ma may possibly be interpreted as the time of extraction of the magma from the mantle, as suggested by positive µNd(850) values (+2,76) from an Adrianópolis sample. An µNd values versus 87Sr/86Sr diagram shows that each studied occurrence plots in a different position, following approximately the mantle array trend, the most primitive being the metabasite of Adrianópolis, near the MORB field, and the most enriched being the amphibolite from the same region, near the enriched mantle and or crustal contamination field. The distinct difference in isotopic signatures between the amphibolite and metabasite suggests derivation from different mantle sources, as supported by available geochemical data. Nevertheless, some of the samples show isotopic evidence of the action of metamorphism, crustal contamination and weathering.

A Report on a Biotite‐Calcic Hornblende Geothermometer

Abstract This paper presents a biotite‐calcic hornblende geothermometer which was empirically calibrated based on the garnet‐biotite geothermometer and the garnet‐plagioclase‐hornblende‐quartz geobarometer, in the ranges of 560–800°C (T) and 0.26–1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe‐Mg‐AlVI‐Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe‐Mg binary solid solution. The resulting thermometer may rebuild the input garnet‐biotite temperatures well within an uncertainty of ±50°C. Errors of ±0.2 GPa for input pressure, along with analytical errors of ±5% for the relevant mineral compositions, may lead to a random error of ±16°C for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assumed that there is a ferric iron content of 11.6% in biotite, and that the iron content in calcic hornblende may be calculated according to the method of Dale et al. (2000). This thermometer can be used for medium‐ to high‐grade metabasites and metapelites.

The origin of the Sierra de Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain

Hollows in the Sierra de Aracena, part of western sector of Sierra Morena region (Huelva, Spain), are geoecologically unusual macroforms. They are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation. Paleosols with very dark colours, a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows. An evolutionary model is proposed for the hollows, involving differential weathering during the Mesozoic on plutonic and amphibolitic rocks, alpine tectonic activity followed by Quaternary erosion and exhumation leading to formation of erosional terraces.