Most Granitoid Rocks are Cumulates: Deductions from Hornblende Compositions and 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.

Download Full-text

Hornblende as a tool for assessing mineral-melt equilibrium and recognition of crystal accumulation

American Mineralogist ◽

10.2138/am-2020-6972 ◽

2020 ◽

Vol 105 (1) ◽

pp. 77-91 ◽

Cited By ~ 2

Author(s):

Kevin Werts ◽

Calvin G. Barnes ◽

Valbone Memeti ◽

Barbara Ratschbacher ◽

Dustin Williams ◽

...

Keyword(s):

Trace Elements ◽

Mixed Crystal ◽

Magma Mixing ◽

Volcanic Rocks ◽

Intrusive Complex ◽

Bulk Rock ◽

Wide Range ◽

Compositional Variability ◽

Crystal Accumulation ◽

Glass Compositions

Abstract Bulk-rock compositions are commonly used as proxies for melt compositions, particularly in silicic plutonic systems. However, crystal accumulation and/or melt loss may play an important role in bulk-rock compositional variability (McCarthy and Hasty 1976; McCarthy and Groves 1979; Wiebe 1993; Wiebe et al. 2002; Collins et al. 2006; Deering and Bachmann 2010; Miller et al. 2011; Vernon and Collins 2011; Lee and Morton 2015; Lee et al. 2015; Barnes et al. 2016a; Schaen et al. 2018). Recognizing and quantifying the effects of crystal accumulation and melt loss in these silicic systems is challenging. Hornblende-melt Fe/Mg partitioning relationships and hornblende (Hbl) chemometry are used here to test for equilibrium with encompassing bulk-rock and/or glass compositions from several plutonic and volcanic systems. Furthermore, we assess the extent to which these tests can be appropriately applied to Hbl from plutonic systems by investigating whether Hbl from the long-lived (~10 Ma) Tuolumne Intrusive Complex preserves magmatic crystallization histories. On the basis of regular zoning patterns, co-variation of both fast- and slow-diffusing trace elements, Hbl thermometry, and compositional overlap with volcanic Hbl we conclude that Hbl from plutons largely preserve records supporting the preservation of a magmatic crystallization history, although many compositional analyses yield calculated temperatures <750 °C, which is unusual in volcanic Hbl. Hornblende is only rarely in equilibrium with host plutonic bulk-rock compositions over a wide range of SiO2 contents (42–78 wt%). Hornblende chemometry indicates that the majority of Hbl from the plutonic systems investigated here is in equilibrium with melts that are typically more silicic (dacitic to rhyolitic in composition) than bulk-rock compositions. These results are consistent with crystal accumulation and/or loss of silicic melts within middle- to upper-crustal plutons. Although the processes by which melts are removed from these plutonic systems is uncertain, it is evident that these melts are either redistributed in the crust (e.g., leucogranite dikes, plutonic roofs, etc.) or are instead erupted. In contrast, Hbl from volcanic rocks is more commonly in equilibrium with bulk-rock and glass compositions. In most cases, where Hbl is out of equilibrium with its host glass, the glasses are more evolved than the calculated melts indicating crystallization from a less fractionated melt and/or mixed crystal populations. Where Hbl is not in equilibrium with volcanic bulk-rocks, the bulk-rock compositions are typically more mafic than the calculated melts. In some intermediate volcanic samples, the occurrence of wide-ranges of calculated melt compositions is indicative of magma mixing. The general absence of Hbl with temperatures <750 °C from volcanic systems suggests that magmatic mushes below this temperature are unlikely to erupt. Our results indicate that bulk-rock compositions of granitic plutonic rocks only rarely approximate melt compositions and that the possibility of crystal accumulation and/or melt loss cannot be ignored. We suggest that detailed assessments of crystal accumulation and melt loss processes in magmatic systems are crucial to evaluating magma differentiation processes and discerning petrogenetic links between plutonic and volcanic systems.

Download Full-text

Immiscibility and the origin of ladder structures, mafic layering, and schlieren in plutons

Geology ◽

10.1130/g47634.1 ◽

2020 ◽

Vol 49 (1) ◽

pp. 86-90

Author(s):

Allen F. Glazner ◽

John M. Bartley ◽

Bryan S. Law

Keyword(s):

Field Strength ◽

High Field ◽

Liquid Immiscibility ◽

Granitic Rocks ◽

Calc Alkaline ◽

High Field Strength ◽

Incompatible Elements ◽

Modal Layering ◽

Crystal Accumulation ◽

Chemical Trends

Abstract Granitic plutons worldwide contain ladder structures (LSs) that consist of nested trough-shaped layers alternating between mafic and felsic compositions. LSs and other forms of modal layering have been attributed to crystal accumulation, but their chemical trends differ greatly from those of cumulates and are discordant with chemical variations of their granitic hosts. Mafic layers reach extreme enrichments in transition metals, high-field-strength elements, and incompatible elements, and are extremely depleted in Si and Al. These geochemical characteristics are difficult to explain by crystal accumulation and conflict with sequences of phase appearance during crystallization. They are characteristic of liquid immiscibility, which is an accepted process in the genesis of tholeiitic and alkalic rocks. We propose that ladder structures and other forms of modal layering are markers of immiscibility in calc-alkaline granitic rocks.

Download Full-text

A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex

Geosphere ◽

10.1130/ges02233.1 ◽

2021 ◽

Author(s):

C.G. Barnes ◽

K. Werts ◽

V. Memeti ◽

S.R. Paterson ◽

R. Bremer

Keyword(s):

Trace Element ◽

Volcanic Rocks ◽

Granitic Rocks ◽

Intrusive Complex ◽

Bulk Rock ◽

Rhyolitic Magma ◽

Magma Reservoirs ◽

Mineral Compositions ◽

Rock Analysis ◽

End Members

The widespread occurrence of mafic magmatic enclaves (mme) in arc volcanic rocks attests to hybridization of mafic-intermediate magmas with felsic ones. Typically, mme and their hosts differ in mineral assemblage and the compositions of phenocrysts and matrix glass. In contrast, in many arc plutons, the mineral assemblages in mme are the same as in their host granitic rocks, and major-element mineral compositions are similar or identical. These similarities lead to difficulties in identifying mixing end members except through the use of bulk-rock compositions, which themselves may reflect various degrees of hybridization and potentially melt loss. This work describes the variety of enclave types and occurrences in the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex and then focuses on textural and mineral composition data on five porphyritic mme from the eHD. Specifically, major- and trace-element compositions and zoning patterns of plagioclase and hornblende were measured in the mme and their adjacent host granitic rocks. In each case, the majority of plagioclase phenocrysts in the mme (i.e., large crystals) were derived from a rhyolitic end member. The trace-element compositions and zoning patterns in these plagioclase phenocrysts indicate that each mme formed by hybridization with a distinct rhyolitic magma. In some cases, hybridization involved a single mixing event, whereas in others, evidence for at least two mixing events is preserved. In contrast, some hornblende phenocrysts grew from the enclave magma, and others were derived from the rhyolitic end member. Moreover, the composition of hornblende in the immediately adjacent host rock is distinct from hornblende typically observed in the eHD. Although primary basaltic magmas are thought to be parental to the mme, little or no evidence of such parents is preserved in the enclaves. Instead, the data indicate that hybridization of already hybrid andesitic enclave magmas with rhyolitic magmas in the eHD involved multiple andesitic and rhyolitic end members, which in turn is consistent with the eHD representing an amalgamation of numerous, compositionally distinct magma reservoirs. This conclusion applies to enclaves sampled <30 m from one another. Moreover, during amalgamation of various rhyolitic reservoirs, some mme were evidently disrupted from a surrounding mush and thus carried remnants of that mush as their immediately adjacent host. We suggest that detailed study of mineral compositions and zoning in plutonic mme provides a means to identify magmatic processes that cannot be deciphered from bulk-rock analysis.

Download Full-text

A simple and robust method for calculating temperatures of granitoid magmas

Mineralogy and Petrology ◽

10.1007/s00710-021-00769-5 ◽

2021 ◽

Author(s):

Meng Duan ◽

Yaoling Niu ◽

Pu Sun ◽

Shuo Chen ◽

Juanjuan Kong ◽

...

Keyword(s):

Empirical Equation ◽

Rock Sample ◽

Saturation Temperature ◽

Theory And Practice ◽

Bulk Rock ◽

Experimental Calibration ◽

Granitoid Rocks ◽

Modal Abundance ◽

Input Variables ◽

Zircon Saturation

AbstractCalculating the temperatures of magmas from which granitoid rocks solidify is a key task of studying their petrogenesis, but few geothermometers are satisfactory. Zircon saturation thermometry has been the most widely used because it is conceptually simple and practically convenient, and because it is based on experimental calibrations with significant correlation of the calculated zircon saturation temperature (TZr) with zirconium (Zr) content in the granitic melt (i.e., TZr ∝ ZrMELT). However, application of this thermometry to natural rocks can be misleading, resulting in the calculated TZr having no geological significance. This thermometry requires Zr content and a compound bulk compositional parameter M of the melt as input variables. As the Zr and M information of the melt is not available, petrologists simply use bulk-rock Zr content (ZrBULK-ROCK) and M to calculate TZr. In the experimental calibration, TZr shows no correlation with M, thus the calculated TZr is only a function of ZrMELT. Because granitoid rocks represent cumulates or mixtures of melt with crystals before magma solidification and because significant amount Zr in the bulk-rock sample reside in zircon crystals of varying origin (liquidus, captured or inherited crystals) with unknown modal abundance, ZrBULK-ROCK cannot be equated with ZrMELT that is unknown. Hence, the calculated magma temperatures TZr using ZrBULK-ROCK have no significance in both theory and practice. As an alternative, we propose to use the empirical equation $$T_{SiO_{2}}$$ T S i O 2 (°C) = -14.16 × SiO2 + 1723 for granitoid studies, not to rely on exact values for individual samples but focus on the similarities and differences between samples and sample suites for comparison. This simple and robust thermometry is based on experimentally determined phase equilibria with T ∝ 1/SiO2.

Download Full-text

Ore Genesis of the Takatori Tungsten–Quartz Vein Deposit, Japan: Chemical and Isotopic Evidence

Minerals ◽

10.3390/min11070765 ◽

2021 ◽

Vol 11 (7) ◽

pp. 765

Author(s):

Yuichi Morishita ◽

Yoshiro Nishio

Keyword(s):

Oxygen Isotope ◽

Early Stage ◽

Isotope Ratios ◽

Equilibrium Temperature ◽

Granitic Rocks ◽

Granitic Magma ◽

Lithium Isotope ◽

Vein Deposit ◽

Pressure Independent ◽

Isotope Equilibrium

The Takatori hypothermal tin–tungsten vein deposit is composed of wolframite-bearing quartz veins with minor cassiterite, chalcopyrite, pyrite, and lithium-bearing muscovite and sericite. Several wolframite rims show replacement textures, which are assumed to form by iron replacement with manganese postdating the wolframite precipitation. Lithium isotope ratios (δ7Li) of Li-bearing muscovite from the Takatori veins range from −3.1‰ to −2.1‰, and such Li-bearing muscovites are proven to occur at the early stage of mineralization. Fine-grained sericite with lower Li content shows relatively higher δ7Li values, and might have precipitated after the main ore forming event. The maximum oxygen isotope equilibrium temperature of quartz–muscovite pairs is 460 °C, and it is inferred that the fluids might be in equilibrium with ilmenite series granitic rocks. Oxygen isotope ratios (δ18O) of the Takatori ore-forming fluid range from +10‰ to +8‰. The δ18O values of the fluid decreased with decreasing temperature probably because the fluid was mixed with surrounding pore water and meteoric water. The formation pressure for the Takatori deposit is calculated to be 160 MPa on the basis of the difference between the pressure-independent oxygen isotope equilibrium temperature and pressure-dependent homogenization fluid inclusions temperature. The ore-formation depth is calculated to be around 6 km. These lines of evidence suggest that a granitic magma beneath the deposit played a crucial role in the Takatori deposit formation.

Download Full-text

Origin of chromitites in the Esker Intrusive Complex, Ring of Fire Intrusive Suite, as revealed by chromite trace element chemistry and simple crystallization models

10.4095/328981 ◽

2021 ◽

Author(s):

J M Brenan ◽

K Woods ◽

J E Mungall ◽

R Weston

Keyword(s):

Partition Coefficients ◽

Fluid Dynamic ◽

Experimental Studies ◽

Wall Rock ◽

Major And Trace Elements ◽

Iron Formation ◽

Intrusive Complex ◽

Banded Iron Formation ◽

Magma Flow ◽

Intrusive Suite

To better constrain the origin of the chromitites associated with the Esker Intrusive Complex (EIC) of the Ring of Fire Intrusive Suite (RoFIS), a total of 50 chromite-bearing samples from the Black Thor, Big Daddy, Blackbird, and Black Label chromite deposits have been analysed for major and trace elements. The samples represent three textural groups, as defined by the relative abundance of cumulate silicate phases and chromite. To provide deposit-specific partition coefficients for modeling, we also report on the results of laboratory experiments to measure olivine- and chromite-melt partitioning of V and Ga, which are two elements readily detectable in the chromites analysed. Comparison of the Cr/Cr+Al and Fe/Fe+Mg of the EIC chromites and compositions from previous experimental studies indicates overlap in Cr/Cr+Al between the natural samples and experiments done at &gt;1400oC, but significant offset of the natural samples to higher Fe/Fe+Mg. This is interpreted to be the result of subsolidus Fe-Mg exchange between chromite and the silicate matrix. However, little change in Cr/Cr+Al from magmatic values, owing to the lack of an exchangeable reservoir for these elements. A comparison of the composition of the EIC chromites and a subset of samples from other tectonic settings reveals a strong similarity to chromites from the similarly-aged Munro Township komatiites. Partition coefficients for V and Ga are consistent with past results in that both elements are compatible in chromite (DV = 2-4; DGa ~ 3), and incompatible in olivine (DV = 0.01-0.14; DGa ~ 0.02), with values for V increasing with decreasing fO2. Simple fractional crystallization models that use these partition coefficients are developed that monitor the change in element behaviour based on the relative proportions of olivine to chromite in the crystallizing assemblage; from 'normal' cotectic proportions involving predominantly olivine, to chromite-only crystallization. Comparison of models to the natural chromite V-Ga array suggests that the overall positive correlation between these two elements is consistent with chromite formed from a Munro Township-like komatiitic magma crystallizing olivine and chromite in 'normal' cotectic proportions, with no evidence of the strong depletion in these elements expected for chromite-only crystallization. The V-Ga array can be explained if the initial magma responsible for chromite formation is slightly reduced with respect to the FMQ oxygen buffer (~FMQ- 0.5), and has assimilated up to ~20% of wall-rock banded iron formation or granodiorite. Despite the evidence for contamination, results indicate that the EIC chromitites crystallized from 'normal' cotectic proportions of olivine to chromite, and therefore no specific causative link is made between contamination and chromitite formation. Instead, the development of near- monomineralic chromite layers likely involves the preferential removal of olivine relative to chromite by physical segregation during magma flow. As suggested for some other chromitite-forming systems, the specific fluid dynamic regime during magma emplacement may therefore be responsible for crystal sorting and chromite accumulation.

Download Full-text

Protolith-Related Thermal Controls on the Decoupling of Sn and W in Sn-W Metallogenic Provinces: Insights from the Nanling Region, China

Economic Geology ◽

10.5382/econgeo.4669 ◽

2019 ◽

Vol 114 (5) ◽

pp. 1005-1012 ◽

Cited By ~ 14

Author(s):

Shunda Yuan ◽

Anthony E. Williams-Jones ◽

Rolf L. Romer ◽

Panlao Zhao ◽

Jingwen Mao

Keyword(s):

South China ◽

Bulk Rock ◽

Rare Metals ◽

The West ◽

Metallogenic Province ◽

Peraluminous Granites ◽

Microgranular Enclaves ◽

Mafic Microgranular Enclaves ◽

The World ◽

Zircon Saturation

Abstract The Nanling region of South China hosts the largest W-Sn metallogenic province in the world, accounting for more than 54% of global tungsten resources as well as important resources of tin and rare metals. An important feature of this province, which is shared by a number of other W-Sn metallogenic provinces, is that W deposits occur separately from Sn and Sn-W deposits, with the latter concentrated in the western part of the region (especially along the deep, NE-trending Chenzhou-Linwu fault) and the W deposits to the east of them. All the deposits are associated with ilmenite series, peraluminous granites. However, the granites associated with the Sn and Sn-W deposits can be distinguished from the W granites by their higher bulk-rock εNd values and their higher zircon εHf values. Most importantly, the Sn and Sn-W granites are characterized by higher zircon saturation temperatures (800 ± 20°C) than the W granites (650°–750°C). The Sn and Sn-W granites also contain abundant mantle-derived mafic microgranular enclaves, whereas such enclaves are rare in the W granites. A model is proposed in which the protolith to the W granites released W to the melt as a result of the breakdown of muscovite. The temperature of melting, however, was too low for biotite to melt. In the west, particularly along the Chenzhou-Linwu fault (the location of the Sn and Sn-W deposits), higher temperatures enabled the breakdown of both muscovite and biotite and the consequent release of both Sn and W to form Sn and Sn-W granites. This model, which is based on differences in the protolith melting temperature and thus mobilization temperatures for Sn and W, is potentially applicable to any Sn-W metallogenic province in which the Sn and Sn-W deposits are spatially separated from the W deposits.

Download Full-text

Reconciling zircon and monazite thermometry constrains H2O content in granitic melts

10.5194/egusphere-egu2020-12038 ◽

2020 ◽

Author(s):

Silvia Volante ◽

William Collins ◽

Chris Spencer ◽

Eleanore Blereau ◽

Amaury Pourteau ◽

...

Keyword(s):

Water Content ◽

Volcanic Rocks ◽

Saturation Temperature ◽

Granitic Rocks ◽

Zircon Geochronology ◽

Bulk Rock ◽

Independent Evidence ◽

Water Values ◽

Host Rocks ◽

Eastern Domain

In this contribution, we compare and test the reliability of zircon and monazite thermometers and suggest a new and independent method to constrain the H2O content in granitic magmas from coeval zircon and monazite minerals. We combine multi-method single-mineral thermometry (bulk-rock zirconium saturation temperature (Tzr), Ti-in-zircon (T(Ti-zr)) and monazite saturation temperature (Tmz)) with thermodynamic modelling to estimate water content and P&#8211;T conditions for strongly-peraluminous (S-type) granitoids in the Georgetown Inlier, NE Queensland. These granites were generated within ~30 km thick Proterozoic crust, and emplaced during regional extension associated with low-pressure high-temperature (LP&#8211;HT) metamorphism.SHRIMP U&#8211;Pb monazite and zircon geochronology indicates synchronous crystallization ages of c. 1550 Ma for granitic rocks emplaced at different crustal levels&#8212;from the eastern deep crustal domain (P = 6&#8211;9 kbar), through the middle crustal domain (P = 4&#8211;6 kbar), to the western upper crustal domain (P = 0&#8211;3 kbar).Bulk-rock Tzr and T(Ti-zr) yielded magma temperature estimates for the eastern domain of ~800&#176;C and ~910&#8211;720&#176;C, respectively. Magma temperatures in the central and western domains were ~730&#176;C (Tzr) and ~870&#8211;750&#176;C (T(Ti-zr)) in the central domain, and ~810&#176;C (Tzr) and ~890&#8211;720&#176;C (T(Ti-zr)) in the western domain, respectively. These temperature estimates were compared with P&#8211;T conditions recorded in the host rocks to determine if the magmas had equilibrated thermally with the crust. Similar temperatures were obtained for the middle and lower crust suggesting that the associated magmas thermally equilibrated at their respective depths, whereas the sub-volcanic rocks were, as expected, significantly hotter than the adjacent crust.By plotting the results on a P&#8211;T&#8211;XH2O petrogenetic grid, and assuming adiabatic ascent through the crust, the sub-volcanic magmas appear to be drier (~3 wt% H2O) than the granitic magmas (~7 wt% H2O) which formed at greater depth. Monazite saturation temperatures (which depends on the water content, light&#8211;REE content and composition of the granitic melt), are in agreement with the zircon thermometers only if water values of ~3 wt% H2O and ~7 wt% H2O are assumed for the upper crustal magmas and deeper magmas, respectively. Moreover, melt compositions extracted from a modelled pseudosection of a sillimanite-bearing metapelite, which was interpreted to be the typical source rock for the surrounding granites (P=5 kbar and T=690&#176;C&#8211;850&#176;C), show comparable water content values.The Tmz results provide independent evidence for the H2O content in magmas, and we suggest that reconciling Tzr with Tmz is a new and independent way of constraining H2O content in granitic magmas.

Download Full-text

Composition, age, and origin of granitoid rocks in the Davin Lake area, Rottenstone Domain, Trans-Hudson Orogen, northern Saskatchewan

Canadian Journal of Earth Sciences ◽

10.1139/e04-067 ◽

2005 ◽

Vol 42 (4) ◽

pp. 599-633 ◽

Cited By ~ 2

Author(s):

D Barrie Clarke ◽

Andrew S Henry ◽

Mike A Hamilton

Keyword(s):

Granitic Rocks ◽

Lake Area ◽

Granitoid Magmatism ◽

Metasedimentary Rocks ◽

Metavolcanic Rocks ◽

Granitic Magmatism ◽

Deformation Features ◽

High Grade Metamorphism ◽

Granitoid Rocks ◽

Host Rocks

The Rottenstone Domain of the Trans-Hudson orogen is a 25-km-wide graniticmigmatitic belt lying between the La Ronge volcanicplutonic island arc (18901830 Ma) to the southeast and the ensialic Wathaman Batholith (1855 Ma) to the northwest. The Rottenstone Domain consists of three lithotectonic belts parallel to the orogen: (i) southeast gently folded migmatized quartzo-feldspathic metasedimentary and mafic metavolcanic rocks intruded by small concordant and discordant white tonalitemonzogranite bodies; (ii) central intensely folded and migmatized metasedimentary rocks and minor metavolcanic rocks intruded by largely discordant, xenolith-rich, pink aplite-pegmatite monzogranite bodies; and (iii) northwest steeply folded migmatized metasedimentary rocks cut by subvertical white tonalitemonzogranite sheets. Emplacement of granitoid rocks consists predominantly of contiguous, orogen-parallel, steeply dipping, syntectonic and post-tectonic sheets with prominent magmatic schlieren bands, overprinted by parallel solid-state deformation features. The white granitoid rocks have A/CNK (mol Al2O3/(mol CaO + Na2O + K2O)) = 1.141.22, K/Rb ≈ 500, ΣREE (sum of rare-earth elements) < 70 ppm, Eu/Eu* > 1, 87Sr/86Sri ≈ 0.7032, and εNdi ≈ 2. The pink monzogranites have A/CNK = 1.111.16, K/Rb ≈ 500, ΣREE > 90 ppm, Eu/Eu* < 1, 87Sr/86Sri ≈ 0.7031, and εNdi ≈ 2. The white granitoid rocks show a wider compositional range and more compositional scatter than the pink monzogranites, reflecting some combination of smaller volume melts, less homogenization, and less control by crystalmelt equilibria. All metavolcanic, metasedimentary, and granitic rocks in the Rottenstone Domain have the distinctive geochemical signatures of an arc environment. New sensitive high-resolution ion microprobe (SHRIMP) UPb geochronology on the Rottenstone granitoid rocks reveals complex growth histories for monazite and zircon, variably controlled by inheritance, magmatism, and high-grade metamorphism. Monazite ages for the granitoid bodies and migmatites cluster at ~1834 and ~1814 Ma, whereas zircon ages range from ~2480 Ma (rare cores) to ~19001830 Ma (cores and mantles), but also ~18181814 Ma for low Th/U recrystallized rims, overgrowths, and rare discrete euhedral prisms. These results demonstrate that at least some source material for the granitic magmas included earliest Paleoproterozoic crust (Sask Craton?), or its derived sediments, and that Rottenstone granitic magmatism postdated plutonism in the bounding La Ronge Arc and Wathaman Batholith. We estimate the age of terminal metamorphism in the Davin Lake area to be ~1815 Ma. Petrogenetically, the Rottenstone migmatites and granitoid rocks appear, for the most part, locally derived from their metasedimentary and metavolcanic host rocks, shed from the La Ronge Arc, Sask Craton, and possibly the Hearne Craton. The Rottenstone Domain was the least competent member in the overthrust stack and probably underwent a combination of fluid-present melting and fluid-absent decompression melting, resulting in largely syntectonic granitoid magmatism ~18351815 Ma, analogous to granite production in the High Himalayan gneiss belt.

Download Full-text