scholarly journals Reassessing zircon-monazite thermometry with thermodynamic modelling: insights from the Georgetown igneous complex, NE Australia

2020 ◽  
Vol 175 (12) ◽  
Author(s):  
S. Volante ◽  
W. J. Collins ◽  
E. Blereau ◽  
A. Pourteau ◽  
C. Spencer ◽  
...  
Keyword(s):  
Saturation Temperature ◽  
Thermodynamic Modelling ◽  
Accessory Mineral ◽  
Compositional Variation ◽  
Upper Crust ◽  
Middle Crust ◽  
Deep Crust ◽  
Granite Magmatism ◽  
Zircon Thermometry ◽  
Zircon Saturation

AbstractAccessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated.

scholarly journals Transient rhyolite melt extraction to produce a shallow granitic pluton

2021 ◽  
Vol 7 (21) ◽  
pp. eabf0604
Author(s):  
Allen J. Schaen ◽  
Blair Schoene ◽  
Josef Dufek ◽  
Brad S. Singer ◽  
Michael P. Eddy ◽  
...  
Keyword(s):  
Time Scales ◽  
Explosive Eruptions ◽  
Upper Crust ◽  
Saturation Model ◽  
Granitic Pluton ◽  
Melt Extraction ◽  
High Silica ◽  
Melt Segregation ◽  
Zircon Saturation

Rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. However, the time scales over which these systems evolve are mainly understood through erupted deposits, limiting confirmation of this connection. Exhumed plutons that preserve a record of high-silica melt segregation provide a critical subvolcanic perspective on rhyolite generation, permitting comparison between time scales of long-term assembly and transient melt extraction events. Here, U-Pb zircon petrochronology and 40Ar/39Ar thermochronology constrain silicic melt segregation and residual cumulate formation in a ~7 to 6 Ma, shallow (3 to 7 km depth) Andean pluton. Thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions. Our findings suggest that ~50 km3 of rhyolitic melt was extracted in ~130 ka, transient pluton assembly that indicates the thermal viability of advanced magma differentiation in the upper crust.

The geology and structure of the mid-crustal Wawa gneiss domain: a key to understanding tectonic variation with depth and time in the late Archean Abitibi–Wawa orogen

1994 ◽  
Vol 31 (7) ◽  
pp. 1064-1080 ◽  
Author(s):  
D. E. Moser
Keyword(s):  
Structural Evolution ◽  
Shear Zones ◽  
Fault Reactivation ◽  
Upper Crust ◽  
Middle Crust ◽  
Tectonic Events ◽  
Granulite Metamorphism ◽  
Stage 1 ◽  
Structural Levels ◽  
East West

The amphibolite-facies central Wawa gneiss domain (CWGD) preserves structures that developed at the mid-crustal level of the ca. 2.7 Ga Abitibi–Wawa orogen in the southern Superior Province. The relative ages of these domainal structures are documented and brackets on their absolute ages established using existing U–Pb age data. Correlation of tectonic events within the CWGD, and comparison of these events with the evolution of other structural levels of the orogen, has led to subdivision of orogenesis into five stages. During stage 1 (2700–2680 Ma), 2.9 and 2.7 Ga rocks were tightly folded and (or) thrusted at all crustal levels in at least one thick-skinned compression event. During stage 2 (2680–2670 Ma), folding and thrusting of Timiskaming-age sediments at high levels of the orogen was thin-skinned and had no effect on CWGD gneisses. During stage 3 (2670–2660 Ma), while the upper crust was relatively stable, a 1 km thick package of volcanics and sediments, the Borden Lake belt, was underthrust northwards to depths of 30 km and in-folded with orthogneiss of the CWGD. During stage 4 (2660–2637 Ma), coeval east–west extension and granulite metamorphism of the middle crust produced gently dipping shear zones that overprinted earlier fold structures in the CWGD and lower structural levels of the orogen. This took place with minimal effect on the upper crust. Stage 5 (2630–2580 Ma) marks a period of east–west shortening and (or) fault reactivation in the Kapuskasing uplift and upper-crustal greenstone belts that allowed penetration of deep-crustal metamorphic fluids into the latter. In general, analysis of the structural evolution of the CWGD indicates that deformation and metamorphism in the middle crust of the Abitibi–Wawa orogen outlasted that at upper-crustal levels, resulting in the generally shallower dips of planar fabrics in the deeper structural levels of the Kapuskasing uplift crustal cross section.

Study on present deep crust deformation in northern and middle of the Red river fault zone by gravity method

2020 ◽  
Author(s):  
JIan Wang
Keyword(s):  
Tibetan Plateau ◽  
Fault Zone ◽  
Deformation Mode ◽  
Gravity Change ◽  
Red River ◽  
Change Rate ◽  
Middle Crust ◽  
Yunnan Region ◽  
Deep Crust ◽  
Red River Fault

<p>Multidisciplinary research shows the Red river fault zone’s (RRFZ) present movement and deformation state has complex segmentation feature. In order to further reveal its deep deformation mode, firstly, we extract tectonic movement gravity change information from mobile gravity measurement data by remove water storage varation and Vertical movement gravity effect; Secondly, together crust density interfaces model with gravity change information, then we can get the NMRFZ’s deformation mode of deep crust, which causes gravity variation.</p><p>The average effect with a 50km radius is calculated for the recent gravity change rate in the Sichuan-Yunnan region, then the background rate field and the residual gravity change rate field are obtained. The trend of -0.66μGal/yr gravity-low-speed change in Sichuan-Yunnan region indicates that there is an inheritance between the gravity field and the uplifting background of the southeastern Tibetan Plateau. The crustal uplift is an important reason for the negative surface gravity changes, but it is mainly related to the deep tectonic environment. There are local positive change zones in the block boundary area, with obvious lateral extrusion and deep mass accumulation. It reflects that under the dynamic environment of the eastward flow of the Tibetan Plateau, the crust of north and middle-south section of the RRFZ are extruded and the underground mass become densification which make the surface gravity raising. The positive gravity changes in up-middle crust are more obvious than lower crust and Moho in Sichuan-Yunnan area. The RRFZ also exhibits a strong demarcation feature as a plate boundary, and the northern segment is the dividing line of gravity positive and negative changes area, while the middle-southern segment and its two sides also showed a wide range of positive change trends, with deep mass continue accumulation.</p><p>The results of crustal deep deformation show that both the upper and the lower crust are obviously demarcated along the 101.5°E boundary, with the west side of the southwest Yunnan descending (moho: -0.05m/yr, upper-middle crust: -0.03m/yr) and east side of Sichuan-Yunnan block rising (moho ： 0.05m/yr，upper-middle crust： 0.02m/yr), which shows that the control effects in depth of the Kangdian crustal axis. The deformation rate of the deep crust in the RRFZ is the largest, the middle-south is next and the south the smallest. Gradual zone between the middle-south segment of the RRFZ and the Chuxiong-Jianshui fault zone shows strong activity and difference in the upper middle crust.</p>

Most Granitoid Rocks are Cumulates: Deductions from Hornblende Compositions and Zircon Saturation

2019 ◽  
Vol 60 (11) ◽  
pp. 2227-2240 ◽  
Author(s):  
Calvin G Barnes ◽  
Kevin Werts ◽  
Vali Memeti ◽  
Katie Ardill
Keyword(s):  
Partition Coefficients ◽  
Granitic Rocks ◽  
Granitic Magma ◽  
Intrusive Complex ◽  
Bulk Rock ◽  
Granitoid Rocks ◽  
Modal Layering ◽  
Zircon Thermometry ◽  
Crystal Accumulation ◽  
Zircon Saturation

Abstract Cumulate processes in granitic magma systems are thought by some to be negligible and by others to be common and widespread. Because most granitic rocks lack obvious evidence of accumulation, such as modal layering, other means of identifying cumulate rocks and estimating proportions of melt lost must be developed. The approach presented here utilizes major and trace element compositions of hornblende to estimate melt compositions necessary for zircon saturation. It then compares these estimates with bulk-rock compositions to estimate proportions of extracted melt. Data from three arc-related magmatic systems were used (English Peak pluton, Wooley Creek batholith, and Tuolumne Intrusive Complex). In all three systems, magmatic hornblende displays core-to-rim decreases in Zr, Hf, and Zr/Hf. This zoning indicates that zircon must have fractionated during crystallization of hornblende, at temperatures greater than 800 °C. This T estimate is in agreement with Ti-in-zircon thermometry, which yields a maximum T estimate of 855 °C. On the basis of this evidence, concentrations of Zr in melts from which hornblende and zircon crystallized were calculated by (1) applying saturation equations to bulk-rock compositions, (2) applying saturation equations to calculated melt compositions, and (3) using hornblende/melt partition coefficients for Zr. The results indicate that melt was lost during crystallization of the granitic magmas, conservatively at least as much as 40 %. These results are in agreement with published estimates of melt loss from other plutonic systems and suggest that bulk-rock compositions of many granitic rocks reflect crystal accumulation and are therefore inappropriate for use in thermodynamic calculations and in direct comparison of potentially consanguineous volcanic and plutonic suites.

U–Pb geochronology, Nd–Sm geochemistry, structural setting, and tectonic significance of Late Devonian and Paleogene intrusions in northern Yukon and northeastern Alaska

2019 ◽  
Vol 56 (6) ◽  
pp. 585-606
Author(s):  
Larry S. Lane ◽  
James K. Mortensen
Keyword(s):  
Partial Melting ◽  
Ocean Basin ◽  
Volcanic Edifice ◽  
Late Devonian ◽  
Upper Crust ◽  
Upward Migration ◽  
Middle Crust ◽  
Structural Setting ◽  
Magmatic Underplating ◽  
Tectonic Significance

A suite of six Devonian granites and one syenite were emplaced into the upper crust of northern Yukon between 364.8 ± 2.7 and 371.2 ± 1.4 Ma. The Bear Mountain syenite and related rhyolite porphyry in adjacent Alaska intruded at 52.3 ± 0.4 and 53.5 ± 0.2 Ma, respectively. A felsic volcaniclastic unit and quartz-phyric sill are newly documented adjacent to the Mount Sedgwick granite. The volcaniclastic unit may indicate the presence of a related volcanic edifice. The presence of xenocrystic zircon grains in most of the intrusions suggests initial emplacement of magmas began 10–20 Myr before final emplacement into the upper crust. A Famennian final intrusion age coincides with Late Devonian encroachment of Ellesmerian deformation into the region. Attendant crustal flexure, or evolving foreland structures, may have facilitated upward migration of the magmas. Geochemistry of the intrusions indicates that the Devonian magmatism was largely derived from partial melting of lower and middle crust, implying widespread mafic magmatic underplating in Middle to Late Devonian time. Only Dave Lord syenite retains evidence of an original mantle geochemical signature. Mantle underplating may have played a role in localizing extension, volcanism, and rifting that led to the Late Devonian opening of the Angayucham ocean basin. The Eocene Bear Mountain pluton is inferred to be a northerly example of widespread Cenozoic within-plate magmatism in Alaska.

Cretaceous granitoids in SW Japan and their bearing on the crust-forming process in the eastern Eurasian margin

1996 ◽  
Vol 87 (1-2) ◽  
pp. 183-191 ◽  
Author(s):  
Takashi Nakajima
Keyword(s):  
Granitic Rocks ◽  
Upper Crust ◽  
Forming Process ◽  
Middle Crust ◽  
Low Level ◽  
High Level ◽  
Back Arc ◽  
The Relationship ◽  
Sw Japan ◽  
Subordinate Amount

ABSTRACT:The Cretaceous granitic rocks and associated regional metamorphic rocks in SW Japan were formed by a Cordilleran-type orogeny. Southwest Japan is regarded as a hypothetical cross-section of the upper to middle crust of the Eurasian continental margin in the Cretaceous, comprising (1) high-level granitoids (called San-yo type) and weakly to unmetamorphosed accretionary complexes that are exposed on the back-arc side and (2) low-level (Ryoke type) granitoids with high-grade metamorphites up to migmatitic gneisses on the forearc side. All these granitoids are of the ilmenite series, and predominantly I-type, with a subordinate amount of garnet- or muscovite-bearing varieties in the Ryoke zone, but none of these contains cordierite. These mineralogical variations are likely to depend more on their slightly peraluminous chemistry rather than the pressure differences during crystallisation.In the eastern part of SW Japan, the granitoids of both levels give K–Ar biotite ages of approximately 65 Ma, whereas the magmatic age of high-level granitoids is approximately 70 Ma, 15 Ma younger than the nearly 85 Ma old lower level granitoids. This implies that the formation of the middle crust started approximately 15 Ma before that of the upper crust. The middle crust material was kept over 500°C for 15–20 Ma after solidification, then it cooled together with the upper crust to 300°C, 6–7 Ma after the formation of the upper crust. The coincidence of cooling history below 500°C of the upper and middle crust may reflect the regional uplift of the crust.The low-level granitoids have higher 87Sr/86Sr initial ratios than those of high-level granitoids in the middle-western part (Chugoku district), but the relationship appears to be opposite in the eastern part. This may imply that the two plutonic series formed by separate magmatic pulses at an interval of c. 15 Ma, even though they are not independent, but rather part of a larger episode of crustal growth.

scholarly journals Significance of variation in extent of recrystallization of zircon in orogenic eclogite

2020 ◽  
Author(s):  
Donna Whitney ◽  
Clementine Hamelin ◽  
Christian Teyssier ◽  
Francoise Roger ◽  
Patrice Rey
Keyword(s):  
Aqueous Fluid ◽  
Preferred Orientation ◽  
Rock Interaction ◽  
Massif Central ◽  
Crystallographic Preferred Orientation ◽  
Time Deformation ◽  
Rock Types ◽  
Deep Crust ◽  
Eclogite Facies ◽  
Zircon Thermometry

<p>Migmatite domes are common structures in orogens, and in some cases are comprised of deeply-sourced crust that experienced lateral and subsequent vertical flow, with ultimate emplacement in the mid/upper crust. The record of the deep-crustal history survives in layers and lenses of refractory rock types within the dominant quartzofeldspathic gneiss. These deep-crustal relics are typically the best archives of pressure-temperature-time-deformation conditions of crustal flow, although it can be difficult to extract information about the duration of deep-crustal residence – such as might accompany lateral flow of deep-crust – because intracrystalline diffusion at protracted high temperatures may erase much of the history and/or minerals may record only the timing of final emplacement and cooling. One possible indicator of deep-crustal history is the extent of recrystallization of zircon that experienced eclogite-facies conditions; the conditions of zircon growth/recrystallization are indicated by REE abundance and results of Ti-in-zircon thermometry. For example, in the eclogite-bearing Montagne Noire migmatite dome of the southern French Massif Central, zircon in eclogite from the core of the dome has been extensively recrystallized under eclogite-facies conditions. In contrast, zircon in eclogite from the margin of the dome experienced very little recrystallization and largely consists of inherited (magmatic) cores with very thin (<20 um) eclogite-facies rims. The two eclogites, which both contain garnet + omphacite + rutile + quartz, record the same age of protolith crystallization (~450 Ma) and high-P metamorphism (~315 Ma), and similar metamorphic conditions (700 ± 20°C, 1.4 ±0.1 GPa). Differences in extent of recrystallization of zircon in the two eclogites may relate to duration at high T and/or extent of interaction with aqueous fluid (ongoing work to obtain in situ oxygen isotope data for zircon and garnet will evaluate the latter for each eclogite). Deformation may have been involved in recrystallization of zircon, but is not the primary factor accounting for the differences in extent of recrystallization; both eclogites were deformed during eclogite-facies metamorphism, as indicated by crystallographic-preferred orientation of omphacite and shape-preferred orientation of rutile. Other variables that are also unlikely to explain differences in these eclogite zircons are differences in host rock chemistry, availability of Zr from decompression reactions involving Zr-bearing minerals, extent of radiation damage, and original crystal size. The two most likely explanations for variations in zircon recrystallization are duration at high-T and extent of fluid-rock interaction. In the case of the former, dome-margin eclogite may have had a shorter residence time in the deep crust and was more directly exhumed from a proximal source, whereas the dome-core eclogite may have had a more extended transit in the deep-crust before being exhumed in the steep, median high-strain zone of the migmatite dome.</p>

Crustal-Scale Geology and Fault Geometry Along the Gold-Endowed Matheson Transect of the Abitibi Greenstone Belt

2021 ◽  
Author(s):  
Rasmus Haugaard ◽  
Fabiano Della Justina ◽  
Eric Roots ◽  
Saeid Cheraghi ◽  
Rajesh Vayavur ◽  
...  
Keyword(s):  
Lower Crust ◽  
Greenstone Belt ◽  
Hydrothermal Fluids ◽  
Gravity Models ◽  
Fault System ◽  
Upper Crust ◽  
Magnetotelluric Data ◽  
Middle Crust ◽  
Low Resistivity ◽  
Abitibi Greenstone Belt

Abstract Gold in the Abitibi greenstone belt in the Superior craton, the most prolific gold-producing greenstone terrane in the world, comes largely from complex orogenic mineralizing systems related to deep crustal deformation zones. In order to get a better understanding of these systems, we therefore combined new magnetic, gravity, seismic, and magnetotelluric data with stratigraphic and structural observations along a transect in the Matheson area of the Abitibi greenstone belt to constrain large-scale geologic models of the Archean crust. A high-resolution seismic transect reveals that the well-known Porcupine Destor fault dips shallowly to the south, whereas the Pipestone fault dips steeply to the north. Facing directions and gravity models indicate that these faults are thrust faults where older mafic volcanic rocks overlie a younger sedimentary basin. The depth of the basin reaches ~2 to 2.5 km between these two faults, where it is interpreted to overlie mafic-dominated volcanic substrata. Regional seismic and magnetotelluric surveys image the full crust down to 36-km depth to reveal a heterogeneous architecture. Three crustal-scale layers include a resistive (104–105 Ωm) upper crust of granite-greenstone rocks, a low-resistivity (~10–50 Ωm) middle crust dominated by granitic plutons for which low resistivity is attributed to the presence of graphite, and a low to moderately resistive (50–1,000 Ωm) and seismically homogeneous lower crust interpreted as granulite gneisses. The significant resistivity transition between upper and middle crust is interpreted to be the result of interconnected micrographite grain coating, precipitated from carbon-bearing crustal fluids emplaced during Neoarchean craton stabilization. A major subvertical, seismically transparent, and extremely low resistive (<10 Ωm) corridor connects the lower and middle crust with the upper crust. The geometry of this low-resistivity feature supports its interpretation as a deep-rooted extensional fault system where the corridor acted as a regional-scale conduit for gold-bearing hydrothermal fluids from a ductile source region in the lower crust to the depositional site in the brittle upper crust. We propose that this newly discovered whole crustal corridor focused the hydrothermal fluids into the Porcupine Destor fault in the Matheson region.

scholarly journals The chevkinite group: underestimated accessory phases from a wide range of parageneses

Mineralogia ◽  
2013 ◽  
Vol 44 (3-4) ◽  
pp. 99-114 ◽  
Author(s):  
Bogusław Bagiński ◽  
Ray Macdonald
Keyword(s):  
Critical Role ◽  
Geochemical Modelling ◽  
Accessory Mineral ◽  
Compositional Variation ◽  
Optical Techniques ◽  
Wide Range ◽  
Mineral Assemblages ◽  
Coherent Pairs ◽  
Near Future

AbstractChevkinite-group minerals are widespread in a very wide range of igneous and metamorphic parageneses, forming important components of accessory mineral assemblages. Their presence in a rock may be difficult to establish by standard optical techniques, which has contributed to their importance being underestimated; a combination of SEM and EMPA is recommended here. Currently, there are eleven IMAapproved members of the group but undoubtedly several more will be described in the near future. There is considerable compositional variation in the group, which can be expressed as: REE + M2+C + M3+C = Ca2+ A + Sr + Ti4+C + Zr4+C where A and C are structural sites. Chevkinite-group minerals strongly fractionate geochemically coherent pairs, such as LREE-HREE, Nb-Ta, Zr-Hf and Th-U, and thus play a critical role in geochemical modelling.

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