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1.
Hydrothermal zircon can be used to date fluid-infiltration events and water/rock interaction. At the Boggy Plain zoned pluton (BPZP), eastern Australia, hydrothermal zircon occurs with hydrothermal scheelite, molybdenite, thorite and rutile in incipiently altered aplite and monzogranite. The hydrothermal zircon is texturally distinct from magmatic zircon in the same rocks, occurring as murky-brown translucent 20–50 μm-thick mantles on magmatic cores and less commonly as individual crystals. The hydrothermal mantles are internally textureless in back-scatter electron and cathodoluminescence images whereas magmatic zircon is oscillatory zoned. The age of the hydrothermal zircon is indistinguishable from magmatic zircon, indicating precipitation from a fluid evolved from the magma during the final stages of crystallization. Despite indistinguishable U-Pb isotopic compositions, the trace-element compositions of the hydrothermal and magmatic zircon are distinct. Hydrothermal zircon is enriched in all measured trace-elements relative to magmatic zircon in the same rock, including V, Ti, Nb, Hf, Sc, Mn, U, Y, Th and the rare-earth elements (REE). Chondrite-normalized REE abundances form two distinct pattern groupings: type-1 (magmatic) patterns increase steeply from La to Lu and have Ce and Eu anomalies—these are patterns typical for unaltered magmatic zircon in continental crust rock types; type-2 (hydrothermal) patterns generally have higher abundances of the REE, flatter light-REE patterns [(Sm/La)N = 1.5–4.4 vs. 22–110 for magmatic zircon] and smaller Ce anomalies (Ce/Ce* = 1.8–3.5 vs. 32–49 for magmatic zircon). Type-2 patterns have also been described for hydrothermally-altered zircon from the Gabel Hamradom granite, Egypt, and a granitic dyke from the Acasta Gneiss Complex, Canada.Hadean (∼4.5–4.0 Ga) zircon from the Jack Hills, Western Australia, have variable normalized REE patterns. In particular, the oldest piece of Earth—zircon crystal W74/2-36 (dated at 4.4 Ga)—contains both type-1 and type-2 patterns on a 50 μm scale, a phenomenon not yet reported for unaltered magmatic zircon. In the context of documented magmatic and hydrothermal zircon compositions from constrained samples from the BPZP and the literature, the type-2 patterns in crystal W74/2-36 and other Jack Hills Hadean (JHH) zircon are interpreted as hydrothermally-altered magmatic compositions. An alteration scenario, constrained by isotope and trace-element data, as well as α-decay event calculations, involving fluid/zircon cation and oxygen isotope exchange within partially metamict zones and minor dissolution/reprecipitation, may have occurred episodically for some JHH zircon and at ∼4.27 Ga for zircon W74/2-36. Type-2 compositions in JHH zircon are interpreted to represent localized exchange with a light-REE-bearing, high δ18O (∼6–10‰ or higher) fluid. Thus, a complex explanation involving “permanent” liquid water oceans, large-scale water/rock interaction and plate tectonics in the very early Archean is not necessary as the zircon textures and compositions are simply explained by exchange between partially metamict zircon and a low volume ephemeral fluid.  相似文献   

2.
We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ≈ 0.7, a SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.  相似文献   

3.
Geochronological, geochemical, whole-rock Sr–Nd, and zircon Hf isotopic analyses were carried out on the Jiasha Gabbro, mafic microgranular enclaves (MME) and host Longchahe Granite samples from the Gejiu area in the southeast Yunnan province, SW China, with the aim of characterizing their petrogenesis. Compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin. The Longchahe Granite largely comprises metaluminous quartz monzonite with distinctive K-feldspar megacrysts, but also contains a minor component of peraluminous leucogranite. The MME have spheroidal to elongated/lenticular shapes with sharp, crenulated and occasionally diffuse contacts with the host granite, which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma. Field observations, geochronology, geochemistry, Sr–Nd and zircon Hf isotopic compositions point to a complex petrogenesis for this granite–MME–gabbro association. Zircon 206Pb/238U ages determined by LA-ICP-MS for a mafic enclave, its host granite and the gabbro body are 83.1 ± 0.9 Ma, 83.1 ± 0.4 Ma and 83.2 ± 0.4 Ma, respectively, indicating coeval crystallization of these igneous rock units. Crystal fractionation processes can explain much of the compositional diversity of the Jiasha Gabbro. The geochemical features of the gabbro, such as high Mg# (up to 70) and Cr (up to 327 ppm), enrichment in LILEs (e.g., Rb, Ba, K2O) and LREEs, and depletion in HFSE (e.g., Nb, Ta, Ti), together with initial 87Sr/86Sr ratios of 0.708–0.709 and negative εNd(t) values (−5.23 to −6.45), indicate they were derived from a mantle source that had undergone previous enrichment, possibly by subduction components. The Longchahe Granite has a large range of SiO2 (59.87–74.94 wt%), is distinctly alkaline in composition, and has Sr–Nd–Hf isotopic compositions ((87Sr/86Sr)i > 0.712, εNd(t) = −6.93 to −7.62 and εHf(t) = −5.8 to −9.9) that are indicative of derivation from a crustal source. However, the most primitive rocks of Longchahe Granite are compositionally distinct from any feasible crustal melt. We interpret the spectrum of rock types of the Longchahe Granite to have formed via mixing between crustally derived peraluminous leucogranite magma and mantle-derived magma of similar heritage to the Jiasha Gabbro. We speculate that this mixing event occurred early in the magmatic history of these rocks at relatively high temperature and/or deep in the crust to allow efficient physical mixing of magmas. Saturation and accumulation of K-feldspar and zircon in the mixed magma is invoked to explain the megacrystic K-feldspar and elevated K2O and Zr content of some of the granitic rocks. A later episode of magma mixing/mingling is preserved as the MME that have geochemical and isotopic compositions that, for the most part, are intermediate between the granite and the gabbro. The MME are interpreted to be fractionated melts of mafic magma related to gabbro that were subsequently injected into the cooler, partly crystalline granitic magma. Mingling and mixing processes within the convectively dynamic upper crustal magma chamber resulting in a hybrid (MME) magma. During this second mixing episode, element interdiffusion, rather than bulk physical mixing, is interpreted to be the dominant mixing process.  相似文献   

4.
Lower ocean crust is primarily gabbroic, although 1–2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater–rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform δ18O with an average value of 5.2 ± 0.5‰ (2SD). The average δ18O(Zrc) would be in magmatic equilibrium with unaltered MORB [δ18O(WR) ~ 5.6–5.7‰], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured δ18O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like δ18O(Zrc) requires melting and zircon crystallization prior to significant amounts of water–rock interactions that alter the protolith δ18O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated δ18O (6.0–7.5‰), but such values have not been identified in any zircons from the large sample suite examined here. The difference in δ18O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.  相似文献   

5.
The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.  相似文献   

6.
The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-, P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7–1.3 wt% Li2O, 2–5.5 wt% F, 2.8–4 wt% P2O5 and 0–2.8 wt% B2O3, and their aluminum saturation index ranges from peralkaline to peraluminous (ASILi = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments, pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics, but additional experiments using natural manganotantalite and zircon of relatively pure composition (i.e., close to end member composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model relating the enthalpy of dissolution of manganotantalite to the ASILi of the melt is proposed: ∆H diss (kJ/mol) = 304 × ASILi − 176 in the peralkaline field, and ∆H diss (kJ/mol) = −111 × ASILi + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the melt, i.e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions. A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts.  相似文献   

7.
In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted SHRIMP U–Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-, Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt (quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light (U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon ages from both plutons exhibit bimodal distributions in U–Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newly precipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and 2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U–Pb analyses document the origin of the leucogranite by partial melting of the tonalite. High alkalis (Na2O + K2O), Rb, Nb, HREE, FeOt/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900–950°C), moderate-pressure (<10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction-zone environment. The leucogranite probably formed in a post-collisional realm immediately after accretion of the tonalitic crust.  相似文献   

8.
Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite, 40Ar/39Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km3 a−1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions.  相似文献   

9.
The U-Pb dating of 18 samples, representing the principal rock types of the 4000 km2 Salmi anorthosite-rapakivi granite complex and its satellite Uljalegi pluton, southeastern Baltic (Fennoscandian) Shield, reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years. From oldest to youngest they are: (1) gabbronorite and monzonite at 1546.7 Ma; (2) syenogranite at 1543.4 Ma; (3) early wiborgite and pyterlite at 1540.6–1537.9 Ma; (4) biotite granite and more evolved granite at 1538.4–1535 Ma; (5) late pyterlite at 1535.2 Ma; (6) olivine gabbro and biotite-amphibole granite at 1530 Ma. The resolvable intervals between magmatic episodes are 3.5–5.0 million years. Early wiborgite and pyterlite (3, above) and biotite granite (4, above) probably crystallized from multiple magma intrusions. Age differences of 3.4±1.5 million years between zircon and baddeleyite in olivine gabbro (6, above) are probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the Salmi complex. The ages and chemical features of early and late zircon populations, together with our modeling of magma crystallization and zircon growth, show that the duration of magma crystallization and Pb-diffusion in zircon was short lived and insignificant compared to the precision of dating of about ±1–2 million years. Hence, the range of U-Pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas. Average rate of magma emplacement was about 0.01 km3/year for the most voluminous phase of early biotite-amphibole rapakivi granite, and about 0.0024 km3/year for the Salmi complex as a whole. Compositional changes of the Salmi magmas over time are in agreement with the model of magmatism related to lithospheric extension. Received: 2 August 1996 / Accepted 19 December 1996  相似文献   

10.
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.  相似文献   

11.
The U–Pb ages, REE content, and oxygen isotopic composition of zircon rims developed within a major shear zone in the Kalak Nappe Complex (KNC), Arctic Norway have been determined along with the age of monazite crystals. Different generations of granitic veins have been distinguished based on both field criteria and monazite ages of 446 ± 3 and 424 ± 3 Ma. Within each of these veins, inherited zircon cores are mantled by homogeneous low CL-response zircon rims which yield a range of concordant U–Pb dates of ca. 470–360 Ma. Significant numbers of zircon rims coincide with the timing of monazite crystallization. The zircon rims have moderate light REE enrichment compared to cores, distinctive (Sm/La) n values of less than 12, and La between 0.3 and 10 ppm. This indicates free elemental exchange between newly formed zircon rims and the surrounding matrix. The rims have calculated accumulated alpha-radiation dosages corresponding with a crystalline structure and δ18O values of 1‰. This implies rim crystallization directly from a zirconium-saturated hydrothermal fluid which was modified by some silicate melt. Growth of the zircon rims was prolonged and locally variable due to preferential fluid flow. A third type of zircon can be recognized, forming both rims and cores, with high alpha-radiation doses, and significant enrichment in La, Pr, and Eu. These are interpreted as low-temperature hydrothermally altered metamict zircons. The high volatile input and partial melting in the shear zone favoured prolonged zircon rim growth due to its ability to easily nucleate on inherited seeds. On the other hand, monazite, susceptible to dissolution and re-growth, crystallized in brief episodes, as has been predicted from theoretical phase diagrams. From a regional perspective, these results elucidate cryptic Ar–Ar cooling ages, providing the first record of a Late Ordovician heating and cooling phase within the KNC prior to the climactic Scandian collision.  相似文献   

12.
Synthetic ZrSiO4 and (mildly to strongly radiation-damaged) natural zircon samples were irradiated with 8.8 MeV 4He2+ ions (fluences in the range 1 × 1013–5 × 1016 ions/cm2). For comparison, an additional irradiation experiment was done with 30 MeV 16O6+ ions (fluence 1 × 1015 ions/cm2). The light-ion irradiation resulted in the generation of new (synthetic ZrSiO4) or additional (mildly to strongly metamict natural samples) damage. The maximum extent of the damage is observed in a shallow depth range approximately 32–33 μm (8.8 MeV He) and ~12 μm (30 MeV O) below the sample surface, i.e. near the end of the ion trajectories. These depth values, and the observed damage distribution, correspond well to defect distribution patterns as predicted by Monte Carlo simulations. The irradiation damage is recognised from the notable broadening of Raman-active vibrational modes, lowered interference colours (i.e. decreased birefringence), and changes in the optical activity (i.e. luminescence emission). At very low damage levels, a broad-band yellow emission centre is generated whereas at elevated damage levels, this centre is suppressed and samples experience a general decrease in their emission intensity. Most remarkably, there is no indication of notable structural recovery in pre-damaged natural zircon as induced by the light-ion irradiation, which questions the relevance of alpha-assisted annealing of radiation damage in natural zircon.  相似文献   

13.
Crystallization thermometers for zircon and rutile   总被引：93，自引：20，他引：73  
Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature:
In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T −1:
The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.  相似文献   

14.
Li diffusion in zircon   总被引：2，自引：2，他引：0  
Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0–1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE+3 that might provide charge balance. Diffusion experiments performed in the presence of H2O–CO2 fluid were run in a piston–cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce H2O–CO2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7Li(p,γ)8Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703–1.151°C at 1 atm for experiments run with the spodumene source:
D\textLi = 7.17 ×10 - 7 exp( - 275 ±11 \textkJmol - 1 /\textRT)\textm2 \texts - 1. D_{\text{Li}} = 7.17 \times 10^{ - 7} { \exp }( - 275 \pm 11\,{\text{kJmol}}^{ - 1} /{\text{RT}}){\text{m}}^{2} {\text{s}}^{ - 1}.  相似文献   

15.
The Lengshuiqing area contains several small intrusions made up of peridotite ± quartz diorite ± granite spatially associated with the Gaojiacun pluton (gabbroids + peridotite + diorite). Ni–Cu sulfide ore occur at Lengshuiqing, hosted in peridotite. SHRIMP U–Pb zircon dating produced the ages of 803 ± 4.2 Ma (peridotite), 807 ± 2.6 Ma (oikocrystic hornblende gabbro), 809 ± 4.3 Ma (hornblende gabbronorites) for the Gaojiacun pluton and 807 ± 3.8 Ma (diorite, intrusion I), 817 ± 6.3 Ma (quartz diorite, intrusion II) and 817 ± 5 Ma (peridotite, intrusion 101) for Lengshuiqing. These ages suggest the emplacement of the Gaojiacun pluton later than the intrusions from Lengshuiqing. The olivine from Lengshuiqing does not contain sulfide inclusions and is relatively Ni-rich (1,150–1,550 ppm Ni), suggesting its crystallisation before the sulfide saturation that generated the Ni–Cu deposits. The olivine of the gabbros in the Gaojiacun pluton is Ni-poor (250–800 ppm), which indicates crystallisation from a severely metal-depleted magma after a sulfide saturation event. The olivine in the peridotites from the Gaojiacun pluton has 800–1,150 ppm Ni and contains sulfide inclusions. Moreover, geological evidence suggests the genesis of the peridotites from Gaojiacun in conduits that were ascending through the gabbroids. A sequence of at least three stages of magma emplacement is proposed: (1) Lengshuiqing; (2) gabbroids from Gaojiacun; (3) peridotites from Gaojiacun. Given the age differences, the intrusions at Lengshuiqing and the Gaojiacun pluton might have been produced by different magmatic events.  相似文献   

16.
This study of La Gloria pluton in the Chilean Andes evaluates what information about magmatic conditions can be extracted from minerals in a granitic pluton, despite lower-temperature re-equilibration. The pluton is zoned vertically from granodiorite/quartz monzodiorite to quartz monzonite at the roof, with the uppermost 1500 m showing the strongest modal and compositional trends. This mimics the pattern frequently inferred from zoning in voluminous ignimbrites: a strongly zoned cap overlying a more homogeneous main␣body. The presence of large, euhedral amphibole ± biotite at the chamber margins and roof indicate that water was concentrated there. Biotite and amphibole compositions indicate a roofward increase in magmatic f HF, f HCl and F/Cl ratio, analogous to pre-eruptive volatile gradients recorded in zoned ignimbrites. Hornblende that crystallized directly from the melt in the volatile-rich wall and roof zones yields total-Al solidification pressures of ˜1 kbar, consistent with the estimated 4000 m of cover at the time of emplacement. In the core of the pluton, actinolitic amphibole formed by reaction of melt with early-crystallized clinopyroxene. Plag-cpx cumulate clots in the lower level are interpreted as early crystallizing phases entrained in rising granitic magma. Cores of amphibole phenocrysts in mafic enclaves suggest initial crystallization at pressures of 2–3 kbar. Lower Ti and Al contents of rims and acicular groundmass amphibole, overlapping the composition of amphibole in the host granitoid, indicate that the enclaves equilibrated with the host at the present exposure level in the presence of interstitial melt. A roofward relative increase in fO2 of the magma is recorded by an increasing proportion of Fe-Ti oxides as a fraction of the mafic phases, greater Mn content of ilmenite, and constant or higher Mg/(Mg+Fe) in hornblende and biotite despite declining whole-rock MgO contents. Association␣of subhedral biotite and magnetite with actinolitic amphibole in clots implies a reaction: K-Ti-hb + O2(gas) = bi + mt + actinolitic amph + titanite. Magnetite coexisting with biotite with Fe/(Fe+Mg) = 0.34– 0.40 implies temperatures of equilibration no lower than about 720–750 °C, i.e., late-magmatic rather than subsolidus. Saturation with respect to a water-rich vapor and subsequent diffusive loss of hydrogen may have caused this oxidation trend, which resulted in the most magnesian mafic phases occurring in the most compositionally evolved rocks, opposite to trends in most zoned ignimbrites, which presumably record conditions nearer the liquidus and prior to exsolution of a water-rich vapor. Two-feldspar and Fe-Ti-oxide geothermometers record subsolidus conditions in the pluton and yield higher temperatures for samples from the roof zone, suggesting that slower cooling at deeper levels allowed these minerals to continue to equilibrate to lower temperatures. Individual minerals span wide ranges in composition at any given level of the pluton, from those appropriate for phenocrysts, to those that record conditions well below the solidus. We suggest that the shallow level and isolated position of the pluton led to rapid escape of magmatic volatiles and rapid cooling, thereby preventing development of a long-lived hydrothermal system. Resulting small water/rock ratios may account for why late-magmatic and subsolidus re-equilibration were not pervasive. Received: 23 August 1996 / Accepted: 18 October 1996  相似文献   

17.
We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) n  = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I Sr = 0.70676–0.70708, ε Nd(79 Ma) = −4.4 to −3.3, with T DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.  相似文献   

18.
The luminescence properties of two single zircon crystals from kimberlite of Yakutia have been studied, excited by the DORIS HASYLAB synchrotron, Germany, within energy range from the visible to the soft X-ray region (5–25, 50–200, and 500–620 eV) at temperatures of 300 and 10 K. The luminescence spectra in the range of 2.5 to 6.0 eV and excitation spectra of the main bands have been examined, the physical nature of the luminescence centers has been discussed, and the luminescence properties of a crystal containing growth (radiation) structural defects and a crystal with the same impurities but annealed in air at 1200°C are compared. The zoned structure of the mineral has been considered and the value of the energy gap (E g) in the mineral has been estimated at 7.1 eV. Two groups of luminescence bands caused by impurities of intrinsic (growth, radiation) nature (E max = 2.1, 2.7–2.8, and 3.2–3.3 eV) and matrix luminescence (E max = 4.4−4.7 and 5.4 eV) probably with the participation of excitons were distinguished on the basis of selective excitation of zircon with different synchrotron energies relative to the gap value (E excit < E g, E excit ∼ E g, and E excit ≫ E g). The short-lived component with a response time of 4 ns has been revealed in the afterglow of zircon in the region of 5.4 eV.  相似文献   

19.
Titanium in phengite: a geobarometer for high temperature eclogites   总被引：1，自引：1，他引：0  
Phengite chemistry has been investigated in experiments on a natural SiO2–TiO2-saturated greywacke and a natural SiO2–TiO2–Al2SiO5-saturated pelite, at 1.5–8.0 GPa and 800–1,050°C. High Ti-contents (0.3–3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si + R2+ = AlIV + AlVI) coupled with the substitution: AlVI + Si = Ti + AlIV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si3Al]O10(OH)2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T–P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.  相似文献   

20.
Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO2, K2O and Na2O contents, negative covariations between P2O5, TiO2 and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive εNd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/YbN = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.  相似文献   

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