Country-rock Schist Research Articles

The tectonic implications of a rare metamorphic event in eastern China during the Earth's “middle age”

The Mesoproterozoic Era has long been considered a relatively stable, silent, and even "boring" period in Earth's history, during which the lithosphere was tectonically inactive. However, an increasing amount of evidence suggests that metamorphism and magmatism were much more widespread and intense than previously thought, implying that this period was dynamic. Here we report metamorphism around ca. 1.4 Ga along the southeastern margin of the North China Craton, eastern China. We conduct TIMA, EMPA, and zircon SHRIMP and LA-ICP-MS U-Pb dating on garnet amphibolites and their country rocks. The detailed mineral composition of a representive garnet amphibolite sample is identified and determined by TIMA. Phase equilibria modelling using pseudosections combined with EMPA data indicates the peak metamorphism stage (M 2 ) of the garnet amphibolites at a P-T condition of 10.6-11.2 kbar/820 ℃. SHRIMP Zircon U-Pb analysis yields an age of 1366 ± 17 Ma for the garnet amphibolites, which is slightly younger than that of its country rock schist (1376 ± 22 Ma). Integrating the metamorphic event with geochemical signatures, magmatic records, and sedimentation patterns in analogous geological settings on other continents, our findings collectively demonstrate a protracted and pervasive orogenic process along the periphery of the Columbia supercontinent.

Origin of pegmatitic melts from granitic magmas in the formation of the Jiajika lithium deposit in the eastern Tibetan Plateau

Granitic pegmatites may be genetically related to granites. However, it is still debatable how pegmatitic melts originated from granitic magmas. For example, a possible mechanism of melt–melt immiscibility for the origin of pegmatitic melt has not been well documented. The Jiajika Li deposit in the eastern Tibetan Plateau is a world-class deposit and is hosted in pegmatites, which occurred as dikes surrounding their source granite (two-mica granite). This paper concerns melt inclusions (MIs) in quartz from the granite–marginal pegmatite (GMP) at the roofward contact of the two-mica granite with the country-rock schist, as the MIs from such pegmatite are considered to represent the original pegmatitic melt and hence are ideal for investigating the origin of pegmatitic melts from granitic magmas. The MIs can be classified into H2O–CH4- and H2O–CO2-bearing types, both containing solid phases, including muscovite, albite, and apatite; the H2O–CO2-bearing MIs are usually associated with the H2O–CO2 fluid inclusions. To prevent leakage or decrepitation of these MIs during heating, external pressures were applied by using a hydrothermal diamond-anvil cell. The compositions of muscovite inside the unheated MIs and glass inside the MIs, which was quenched from a homogenous state, were analyzed using electron microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). During heating, the H2O–CH4-bearing MIs were homogenized into an H2O-saturated melt mostly at 750–800 °C, while the H2O–CO2-bearing MIs, when similarly treated, showed H2O–CO2-oversaturated entrapment features. The H2O–CH4-bearing MIs are considered to be trapped from the original pegmatitic melt containing ∼12 wt% H2O with up to ∼1000 ppm Li, ∼1500 ppm Rb, and ∼1800 ppm Cs, at pressures of ∼550–700 MPa. Based on previous MI-homogenization experiments, it has been reported that the microthermometry features and compositions (e.g., H2O contents) of the H2O–CH4-bearing MIs in the GMP quartz are similar to those in an H2O-rich melt, which was immiscible with an H2O-poor melt inside the heated MIs in quartz from the two-mica granite. Accordingly, in this study, we demonstrate that pegmatitic melts could be originated via melt–melt immiscibility of granitic magma, and that the initial exsolution of the pegmatitic melt from granitic magma can occur on a very small scale, as tiny droplets whose sizes may be similar to those of the observed MIs in granite. In addition to the enrichment of Li, B, and P, the enhanced H2O content under low oxygen fugacity condition in the granitic magma, as well as the decrease in pressure following the magma emplacement, could promote the melt–melt immiscibility.

Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

Tourmaline orbicules in peraluminous monzogranites of Argentina: A study case of fluid-rock interaction between leucogranite and country-rock metasediments

Tourmaline orbicules hosted in peraluminous granites are documented worldwide. Seven occurrences were identified in Argentina. Petrography, mineral chemistry, whole-rock geochemistry mass balance and microthermometric studies were performed in orbicules formed at the cupola of a peraluminous A-type leucogranite (Los Riojanos pluton), as well as complementary investigation was achieved in other orbicules of similar geological setting. Mass balance computations in zoned orbicules consistently confirmed immobility of Si both in core and halo, immobility of K and little loss of Al during halo reactions. Elements gained and lost in the schorl-rich core are Fe, Al, Mg, Ti, Ba, Sr, Y and Zr, and Na, K, Rb and Nb, respectively; in the halo, K, Ba, Sr, Y, Zr and locally CaO, were gained, and Fe, Mg, Na, Al, Rb and Nb were lost. The schorl-rich core is enriched in LREE relative to the leucogranite host. A temperature-salinity plot from fluid inclusion data delineates a magmatic-meteoric mixing trend of diluting salinity with descending temperature. Computed δDH20 values from Los Riojanos orbicule schorl suggest magmatic and magmatic-meteoric mixed origins. In Los Riojanos, mass balance constraints suggest that Fe, Mg, Ba, Sr and metallic traces like Zn and V (±Pb) were most likely derived from country-rock schists and gneisses through fluid-rock exchange reactions. A late magmatic-, volatile-rich- fluid exsolution scenario for the formation of orbicules is envisaged. Schorl crystallization was likely delayed to the latest stages of leucogranite consolidation, not only favored by the high diffusivity of B2O3 preferentially partitioned into the exsolved aqueous-rich fluid, but also likely limited to the low availability of Fe and Mg from the scarce granitic biotite, and to the high F− content of the melt. The spatial confination of orbicules to the contact zone granite-metasediments suggests that orbicules were not formed until exsolved fluids reached the boundary with the biotite-rich country-rock.

The age of undeformed dacite intrusions within the Kolaka Fault zone, SE Sulawesi, Indonesia

We present petrologic, geochemical and U–Pb sensitive high resolution ion microprobe (SHRIMP) data from previously undocumented dacite intrusions from the SE Arm of Sulawesi. The dacites occur in a strand of a major fault (the Kolaka Fault) that crosses the SE Arm of Sulawesi and northern Bone Bay. U–Pb SHRIMP dating shows the “Kolaka Dacite” yields zircon grains and overgrowths that range between ca. 4 and 7Ma, indicating active magmatism in SE Sulawesi at this time. The youngest age population (4.4±0.2Ma) from this range is interpreted to be the maximum crystallization age for the dacite. The Kolaka Dacite is undeformed, and so potentially intruded during or after movement within a strand of the Kolaka Fault. The dacites may have otherwise been emplaced passively along existing foliation planes in the country rock schist. Additional U–Pb data were collected from inherited zircons, yielding ages between 8Ma and 1854Ma. We consider that these inherited zircons are xenocrysts, derived from either (1) a partially melted protolith and/or (2) xenocrysts assimilated during ascent of the magma. In either case, the inherited zircons record the age of the basement rocks beneath this part of SE Sulawesi. These inherited zircon cores show that the SE arm of Sulawesi is underlain by Proterozoic or younger material, validating earlier ideas that the crust here was derived from Gondwana.

Geochemical and Isotopic Properties of Fluids from Gold-Bearing and Barren Quartz Veins of the Sovetskoye Gold Deposit (Siberia, Russia)

This paper reports on the comparative study of fluids trapped in inclusions in gold-bearing and barren quartz veins at the Sovetskoye gold deposit, Yeniseisky ridge, Siberia, Russia. The host rocks are greenschist facies metasedimentary rocks of Upper Proterozoic age. Within the deposit, some veins contain 10 to 25 ppm of Au (gold-bearing veins); others contain less than 1 ppm of Au (barren veins). Fluid inclusions trapped in barren veins are predominantly low-salinity-H2O (4.0–8.0 mass % NaCl equiv.) with variable contents of dissolved CO2 (<12.5 mol %), CH4 (<1.3 mol %), and N2 (<1.7 mol %). Homogenization temperatures vary from 200° to 410°C, and the calculated pressures of entrapment are <1.5 kbars. Inclusion fluids trapped in the host schists are predominantly aqueous (97.6–99.3 mol % H2O), with CO2 contents below 2.4 mol percent. The fluids trapped in ore-bearing quartz inclusions are different. Inclusions show higher homogenization temperatures (up to 630°C) and salinity (up to 20–25 mass % NaCl equiv). Calculated pressures range from 0.7 to 2.0 kbars. Bulk gas chromatographic data of fluids extracted from native gold have contents of CO2 that range from 29.3 to 62.0 mol percent, N2 from 2.7 to 13.2 mol percent, and CH4 from 0.0 to 2.8 mol percent. In gold-bearing quartz the contents of CO2 range from 18.0 to 41.0 mol percent, N2 from 0.1 to 0.2 mol percent, and CH4 from 0.3 to 3.0 mol percent. Raman spectroscopic data of fluid from gold-bearing quartz revealed CO2 contents of individual inclusions as high as 98.2 mol percent, N2 up to 66.4 mol percent, and CH4 up to 88.7 mol percent. Fluids of the country-rock schists which host gold-bearing quartz veins are also enriched in CO2 (up to 49.5 mol %). The host rocks for both barren and gold-bearing quartz veins show consistent REE patterns characterized by a negative Eu anomaly. Fluids from barren quartz veins show similar REE patterns although with a less conspicuous negative Eu anomaly, whereas fluids from gold-bearing quartz show a marked positive Eu anomaly. Similar isotopic values of sulfur in pyrite, pyrrhotite, and arsenopyrite disseminated in country rocks ( δ 34S = 14.7–21.6‰), and in barren ( δ 34S = 12.9–19.9‰) and gold-bearing quartz veins ( δ 34S = 13.3–17.9‰) suggest that sulfur in sulfides was derived from the same source, most likely being the adjacent country rocks. CO2 trapped in fluid inclusions of barren and gold-bearing quartz has distinct isotopic compositions ( δ 13C = −8.1 to −10.2 and −4.9 to +5.2 ‰, respectively), and the δ 13C values are positively correlated with Au contents in veins. Helium isotope composition of both types of quartz veins (3He/4He = 0.04–0.05 Ra for gold-bearing veins and 3He/4He = 0.02 Ra for barren veins) indicates a crustal provenance of mineral-forming fluids. The 40Ar/39Ar dating of sericite demonstrates that the barren veins (890-850 Ma) are significantly older than the gold-bearing veins (830-730 Ma). On the basis of their ages, barren veins are interpreted as synmetamorphic. The age of the gold-bearing veins corresponds to the age of the local granitoids (850-720 Ma), suggesting that the origin of the Sovetskoye deposit is related to the tectonic activity syntetctonic magmatism.

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic, mid-crustal, layered, mafic intrusions, emplaced into Proterozoic metaclastic rocks of NE Scotland soon after the thermal peak of static, high-T, low-P regional metamorphism. This gabbro and its associated contact metamorphic rocks illustrate a variety of processes operating during contact anatexis and subsequent melt segregation and extraction. These processes may closely mirror those occurring at much larger scales in the deep crust during high-grade regional metamorphism and the generation of granitic magmas. The emplacement of the Huntly mafic magma resulted in high-grade contact metamorphism and, locally, anatexis of metapelites, leading to the formation of migmatites. The migmatites and country-rock schists were studied to establish the physical conditions of metamorphism and anatexis, the nature of the melting reactions, the compositions of the melts produced, and the extent to which melting was a closedor open-system process. The country-rock schists immediately to the south of the Huntly Complex contain mineral assemblages characteristic of the regional andalusite zone. Thermobarometry of an andalusite schist yields regional metamorphic conditions of 537 42 C and 0 27 0 12 GPa, consistent with previously published P±T estimates. The contact metamorphic rocks include sillimanite hornfelses, metatexites and diatexites. The metatexites consist of cordierite±K-feldspar hornfels melanosomes and K-feldspar-rich garnetiferous leucosomes. The diatexites consist of schollen of fine-grained granoblastic hornfels and metatexite suspended in igneous-textured matrix rocks composed of abundant sub/euhedral garnet, cordierite, plagioclase and, locally, orthopyroxene, with minor interstitial biotite, K-feldspar and quartz. The hornfels melanosomes and schollen retained their structural integrity during partial melting, but the matrix rocks did not. In the highestgrade diatexites, the assemblage Grt ‡ Opx ‡ Crd‡ Hc ‡ Pl characterizes both the hornfels schollen and the sub/euhedral minerals of the matrix rocks. Application of phase equilibria to Opx-bearing rocks yields estimated peak-metamorphic conditions of 900 50 C, 0 45 0 1 GPa and aH2O5 0 3. The pressure estimate implies an emplacement depth of 16 3 km. The prograde P±T path of contact-metamorphic rocks had a low, positive dP/dT slope, indicating that the gabbro intrusion increased the lithostatic load on the country rocks by overplating. Pseudomorph textures involving Al-silicates provide strong evidence that the diatexites evolved from andalusite schists via a sillimanite hornfels stage. Mineralogical changes reflect a sequence of dehydration reactions, followed by fluid-absent partial-melting reactions involving biotite breakdown. It was the fluid-absent reactions that generated the sub/euhedral minerals in the diatexites, as peritectic phases. In many of the highestgrade diatexites, quartz-free anhydrous solid assemblages were produced via reactions such as Bt ‡ Sil ‡ Grt ˆ Crd ‡ Hc ‡ Kfs ‡ melt and Bt‡ Grt ‡ Crd ˆ Opx ‡ Hc ‡ Kfs ‡ melt. Whole-rock major-element geochemical studies indicate that the Opx±Crd hornfelses and diatexite matrices are depleted in Si and K relative to their schist protoliths. Mass-balance calculations indicate that (1) the Opx±Crd hornfels xenoliths represent solid Ca-, Mg-, Fe-, Al-, Na-rich residues left after extraction of 60% melt; (2) the Opx-bearing diatexite matrix rocks are also residual, and represent restite-enriched crystal±liquid mushes left after extraction of 55% melt; (3) the Opx-free diatexite matrix rocks probably represent restite-enriched mushes that retained a higher proportion of residual melt; (4) the anatectic melts were of H2O-undersaturated, peraluminous, JOURNAL OF PETROLOGY VOLUME 44 NUMBER 6 PAGES 995±1029 2003

Geochemical constraints on restite composition and unmixing in the Velay anatectic granite, French Massif Central

The main anatectic granite of the Velay complex is unique among major French Massif Central Hercynian granitoids in that rather than having an entirely lower crustal source, it formed by mixing between partial melts of the meta-igneous lower crust and ‘upper crustal’ country rock schists and orthogneisses. The geochemical variations in the Velay main anatectic granites cannot, however, be explained by mixing alone as their compositions range to lower SiO 2, with higher Al 2O 3, Fe 2O 3 and TiO 2 and lower Na 2O and CaO, than either end member in mixing. The variations are interpreted as being due to the presence of up to 35% restite in minimum melts of country rock compositions. Primary restites form equilibrium assemblages represented by biotite, ilmenite and surmicaceous enclaves which consist of biotite ± apatite, zircon and almandine. The main anatectic granites more rarely contain schist and gneiss enclaves, quartz resisters and plagioclase restites. Secondary restites are mainly represented by cordierite, and possibly K-feldspar, which formed by recrystallisation of primary biotite-rich restites. The unique characteristics of the Velay main anatectic granites are likely to be due, in part, to its late formation close to the end of the Hercynian orogeny. The metasedimentary lower crust may have become too refractory to yield large volumes of melt following partial melting to form the other major Massif Central granitoids. The heat necessary for partial melting at higher crustal levels was transferred from the lower crust by the intrusion of I-type granites and low volume diorites from the mantle. Upper crustal anatexis was mainly controlled by muscovite breakdown reactions (< 830 to 850 °C) and the liberation of water due to the recrystallisation of biotite to cordierite. The temperatures necessary for biotite breakdown were only achieved locally and resulted in the formation of high-LREE granites.

Magma emplacement in a convergent tectonic orogen, southern Revillagigedo Island, southeastern Alaska

The western metamorphic belt of the British Columbia – Alaska coastal orogen consists of lithostratigraphic units assigned to the Taku terrane and the Gravina sequence. On Revillagigedo Island and Cleveland Peninsula in the Ketchikan quadrangle of southeastern Alaska, greenschist- and amphibolite-grade schists of the western metamorphic belt are intruded by mid-Cretaceous tonalite and leucotonalite plutons and sills. A number of these igneous bodies are spatially associated with northwest-trending shear zones developed along thrust faults that formed during the main phase of metamorphism of the schists. Observations of field relations and structural fabric in the igneous rocks and the country-rock schist show the following: (i) Along the southern shore of Revillagigedo Island, the Moth Bay pluton was emplaced along a shear zone. Magmatic and submagmatic flow foliation in the southern border zone of the pluton parallels foliations in the sheared country rock; rocks in the interior of the pluton are generally unfoliated. (ii) At Carroll Point a group of sills intrude along the same shear zone. In some of the sills, tabular plagioclase phenocrysts are oriented parallel to the shear fabric. (iii) Foliation-parallel sills lie within a shear zone that separates Gravina sequence and Taku terrane rocks at Point Francis on Cleveland Peninsula. Older sills within the shear zone are deformed, and younger intrusions are unfoliated and cut across the fabric. Based on these and similar observations elsewhere in the area, we suggest the mid-Cretaceous tonalite and leucotonalite magmas in the Ketchikan quadrangle intruded during the regional compressional deformation that formed the shear zones.

Lamprophyre dike intrusion and the age of the Alpine fault, New Zealand

The orientations of intrusive rocks from a carbonatitic lamprophyre dike swarm and the history of emplacement relative to country-rock schist structures are compatible with intrusion into tension fractures and Riedel shears formed during initiation of the dextral wrench system of the Alpine fault. New U-Pb and Rb-Sr dates indicate a late Oligocene-early Miocene time of intrusion which, in turn, suggests a mid-Tertiary history for propagation of the Alpine fault plate boundary through South Island, New Zealand.

Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece

Small bodies of pyrrhotite, chalcopyrite, minor pentlandite, and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at Tsangli, Eretria, central Greece. Banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding. A magmatic origin has been proposed for similar sulfide masses in the Troodos ophiolite (Panayiotou, 1980). The compositions of the host rocks, chromite, and of the sulfides have been investigated. On average, the sulfide mineralization, recalculated to metal content in 100% sulfide, contains 0.55% Ni, 5.15% Cu, 0.29% Co, 9 ppb Pd, 179 ppb Pt, 16 ppb Rh, 112 ppb Ru, 31 ppb Ir, 58 ppb Os, and 212 ppb Au. These metal contents, particularly the high Cu/(Cu+Ni) ratio of 0.78 and the Pt/(Pd+Pt) ratio of 0.95, are inconsistent with the sulfides having reached equilibrium with their Ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin. The average δ 34S value of the sulfide bodies is +2 while that of a sample of pyrite from country-rock schist is −15.6. These values are inconclusive as to the origin of the sulfur. It is suggested that the sulfides have been precipitated by hydrothermal fluids, possibly those responsible for the serpentinization of the host rocks. The source of the metals may have been the host rocks themselves.

Carbonatites and Fenitization Associated with a Lamprophyric Dike-Swarm Intrusive into Schists of the New Zealand Geosyncline

At Haast River, South Westland, a lamprophyric dike swarm with carbonatitic affinities intrudes schists of the New Zealand Geosyncline. The dike swarm comprises a series of consanguineous members ranging in composition from ultramafic hornblende-mica peridotite, through camptonitic lamprophyre and mafic sodalite-nepheline microsyenite, to sodalite-tinguaite, potassic trachyte, and carbonatite. One carbonatite dike, 1.25 m thick, consists predominantly of the Ba-Mg carbonate, norsethite (Ba 0.96 Mg 0.79 Fe 0.14 Sr 0.02 Mn 0.03 Ca 0.01 (CO 3 ) 2.02 ) with subordinate magnesian siderite, aegirine, albite, pyrite, sphalerite, galena, and monazite. Other carbonatites contain siderite, norsethite-ankerite, and calcite-dolomite-barite assemblages. Metasomatic alteration of the country-rock schist, likened to fenitization, is most pronounced at the margins of alkalic and carbonatite dikes. It involves the introduction of Na 2 O, CO 2 , S, Nb, and Th into the country rock, with possible depletion in K 2 O. A progressive fenitization sequence from quartzofeldspathic schist to aegirine-crossite-albite schist is petrographically described and illustrated. An analysisis presented of crossite, typical of the strongly pleochroic blue amphibole produced during fenitization. The Haast River dike swarm intruded a geosynclinal terrain in immediately postorogenic times, postdating widespread metamorphism and intensive deformation of the Rangitata orogeny by at most about 10 m.y. Similarities between the Haast River occurrence and other carbonatite complexes from geosynclinal areas are briefly discussed.