Evolution and Li Mineralization of the No. 134 Pegmatite in the Jiajika Rare-Metal Deposit, Western Sichuan, China: Constrains from Critical Minerals

The Jiajika rare-metal deposit located in western Sichuan Province (China) is renowned as the largest lithium deposit in Asia, and the No. 134 pegmatite dike is the largest lithium pegmatite under mining conditions in the area. On the basis of a detailed characterization of textures and minerals in the Jiajika No. 134 pegmatite, two zones (the barren Zone Ⅰ and the spodumene Zone Ⅱ) and three subzones (Zone Ⅱ was subdivided into microcrystalline, medium-fine grained and coarse-grained spodumene zones) have been identified. The detailed mineralogical characteristics of lithium minerals and other indicator minerals from each zone were evaluated by EPMA for illustrating the magmatic–hydrothermal evolution and the cooling path of the Jiajika No. 134 pegmatite. From the outer zone inwards, grain size gradually increased, the typical graphic pegmatite zone was absent, and spodumene randomly crystallized throughout nearly the whole pegmatite body. This evidence indicated a Li-saturated melt prior to pegmatite crystallization, which could be the main cause of the super-large-scale Li mineralization of the Jiajika No. 134 pegmatite. A comparison of the Cs content between primary beryl in the Jiajika No. 134 pegmatite and other important Li-Cs-Ta pegmatites in the world indicates that No. 134 pegmatite shows a high degree of fractional crystallization. The evolution of mica species from muscovite to Li-micas from Zone Ⅰ to Zone Ⅱ marks the transition from the magmatic to the hydrothermal stage in pegmatite evolution. The absence of individual lepidolite and the relatively limited scale of alteration of spodumene (<10 vol%) suggest that the activity of the hydrothermal fluids in the system is limited, which contributes to the preservation of the easily altered Li ores and is also an important controlling factor of the super-large-scale Li mineralization of the pegmatite. Spodumene–quartz intergrowth (SQI) usually occurs partly along the rims of the spodumene grains in the Jiajika No. 134 pegmatite. Combined with the pegmatite mineral equilibria, the results of fluid inclusion studies of the pegmatite and the metamorphic conditions in the area, a constrained P-T path of the magmatic–hydrothermal crystallization of the Jiajika No. 134 pegmatite is proposed. The unusual steeply sloped cooling path of the No. 134 pegmatite could be attributed to the fast pressure drop triggered by the intrusion of a pegmatitic melt along the fractures surrounding the Majingzi granite, which could also be the dominant evolution process for other spodumene pegmatites with similar SQI features in the world. The feature of limited internal geochemical fractionation suggested by mineral-scale geochemical analyses of spodumene and micas, combined with the clear textural zoning of the No. 134 pegmatite, can best be ascribed to the effect of undercooling during pegmatite formation. This effect might be one of the non-negligible rules of pegmatite petrogenesis, and would significantly upgrade the potential of Li mineralization by minimizing diffusional Li transfer to the country rocks.

Large strain formulations for host-inclusion systems and their applications to mineral elastic geobarometry

&lt;p&gt;The recent improvement in spectroscopic methods has allowed the detailed characterization of minerals with very high spatial resolution. Such methods allow the accurate estimation of residual pressures in mineral inclusions from exhumed metamorphic rocks. The residual inclusion pressures can be used to recast the pressure conditions during metamorphic recrystallization (e.g. Moulas et al., 2020). The most common assumptions in the aforementioned models are that 1) the rheology of the host-inclusion system is elastic, 2) the pressure was the same in the host and the inclusion phase at the time of recrystallization and, 3) the host and the inclusion can be treated as elastically isotropic phases.&lt;/p&gt;&lt;p&gt;In this work we focus on isotropic host-inclusion systems. Such solutions appear to be sufficient even for anisotropic minerals such as the Quartz-in-Garnet system (Bonazzi et al., 2019; Moulas et al., 2020; Thomas and Spear, 2018). In addition, numerous experimental studies show that mineral Equations-of-State (EoS) are non-linear and, therefore, mechanical solutions which consider linear-elastic host-inclusion systems may be inadequate. We present two analytical solutions for the host-inclusion problem which can be applied in systems under large strain. In the first approach we consider that the volumetric deformation of minerals is non-linear and the deviatoric stresses can be approximated by linear elasticity. The resulting solution is:&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.2823733de70061122311161/sdaolpUECMynit/12UGE&amp;app=m&amp;a=0&amp;c=299da45f483044b63cf0c7f0d5fc23b7&amp;ct=x&amp;pn=gnp.elif&amp;d=1&quot; alt=&quot;&quot; width=&quot;252&quot; height=&quot;95&quot;&gt; (Eq. 1)&lt;/p&gt;&lt;p&gt;where &amp;#916;P is the residual pressure difference, G is the shear modulus, V are the mineral volumes, superscripts h,i indicate host/inclusion and, &amp;#8220;ini&amp;#8221;/&amp;#8221;fin&amp;#8221; indicate initial and final P-T conditions respectively. This result is similar but not identical with a previously published solution (Guiraud and Powell, 2006). However, we demonstrate that Guiraud and Powell&amp;#8217;s (2006) solution is a linearization of this formulation and its accuracy decreases with increasing pressure range.&amp;#160; Finally, we discuss our results in the framework of a newly-derived, fully-non-linear elastic solution that considers the effects of large finite strain in Neo-Hookean materials (Levin et al., 2020). We conclude that, for common mineral barometry applications, the effects of geometrical non linearity are minor and the application of Eq. 1 is sufficient.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Bonazzi, M., Tumiati, S., Thomas, J.B., Angel, R.J., Alvaro, M., 2019. Assessment of the reliability of elastic geobarometry with quartz inclusions. Lithos 350&amp;#8211;351, 105201. https://doi.org/10.1016/j.lithos.2019.105201&lt;/p&gt;&lt;p&gt;Guiraud, M., Powell, R., 2006. P&amp;#8211;V&amp;#8211;T relationships and mineral equilibria in inclusions in minerals. Earth and Planetary Science Letters 244, 683&amp;#8211;694. https://doi.org/10.1016/j.epsl.2006.02.021&lt;/p&gt;&lt;p&gt;Levin, V.A., Podladchikov, Y.Y., Zingerman, K.M., 2020. An exact solution to the Lame problem for a hollow sphere for new types of nonlinear elastic materials in the case of large deformations. European Journal of Mechanics A / Solids (under revision).&lt;/p&gt;&lt;p&gt;Moulas, E., Kostopoulos, D., Podladchikov, Y., Chatzitheodoridis, E., Schenker, F.L., Zingerman, K.M., Pomonis, P., Taj&amp;#269;manov&amp;#225;, L., 2020. Calculating pressure with elastic geobarometry: A comparison of different elastic solutions with application to a calc-silicate gneiss from the Rhodope Metamorphic Province. Lithos 378&amp;#8211;379, 105803. https://doi.org/10.1016/j.lithos.2020.105803&lt;/p&gt;&lt;p&gt;Thomas, J.B., Spear, F.S., 2018. Experimental study of quartz inclusions in garnet at pressures up to 3.0&amp;#160;GPa: evaluating validity of the quartz-in-garnet inclusion elastic thermobarometer. Contributions to Mineralogy and Petrology 173, 42. https://doi.org/10.1007/s00410-018-1469-y&lt;/p&gt;

Timescales of continental subduction: Constraints from ultrahigh-pressure metapelites in the Western Gneiss Region, Norway

&lt;p&gt;The Western Gneiss Region (WGR), Norway is an archetypal continental ultrahigh-pressure (U)HP terrane with an extensive metamorphic history, recording the subduction and subsequent exhumation of continental crust to depths exceeding 120 km. The vast bulk of past work within the WGR has focused on mafic eclogites. In this study, data from rare garnet-kyanite metapelites in (UHP) domains of the WGR is presented. U&amp;#8211;Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and P&amp;#8211;T conditions were calculated by mineral equilibria forward modelling and Zr-in-rutile thermometry. The Ulsteinvik metapelite defines a prograde path that traverses through ~600&amp;#8211;710 &amp;#176;C and ~11&amp;#8211;14 kbar. Minimum peak conditions are ~750 &amp;#176;C and ~2.9 GPa in an inferred garnet-kyanite-coesite-omphacite-muscovite-rutile-quartz-H&lt;sub&gt;2&lt;/sub&gt;O assemblage. Plagioclase-biotite-quartz intergrowths developed after omphacite-phengite-rutile breakdown on the early retrograde path, followed by cordierite-spinel-plagioclase symplectites after garnet-kyanite-biotite, defining a retrograde P&amp;#8211;T point at ~740 &amp;#176;C and ~7 kbar. Late Ordovician-Early Silurian (~470&amp;#8211;440 Ma) zircon and rutile age data in Ulsteinvik pre-dates the major Scandian UHP subduction episode in the WGR, interpreted as recording early Caledonian subduction within the Bl&amp;#229;h&amp;#248; nappe. Monazite and apatite U-Pb geochronology and trace element data suggest exhumation occurred at ~400 Ma. The Fj&amp;#248;rtoft metapelite is a constituent of the Bl&amp;#229;h&amp;#248; nappe. Minimum peak P&amp;#8211;T conditions are ~1.8 GPa and ~750 &amp;#176;C, with poor peak mineral fidelity attributed to extensive retrograde deformation. Negative Eu anomalies in ~423 Ma monazite suggest retrograde conditions were reached [RJT1]&amp;#160;by ~423 Ma. Ulsteinvik and Fj&amp;#248;rtoft may have experienced pre-Scandian subduction together within the Bl&amp;#229;h&amp;#248; nappe, but record dissimilar histories after this. Two potential scenarios are presented: (1) Ulsteinvik resided within the mantle for 20 million-years longer than Fj&amp;#248;rtoft during Scandian subduction, or (2), the samples were exhumed at different times during pre-Scandian subduction of the Bl&amp;#229;h&amp;#248; nappe. The preservation of prograde zoning within Ulsteinvik garnets precludes a long-term residence within the mantle and suggests the latter option. In this scenario, the subducting Bl&amp;#229;h&amp;#248; nappe experienced a degree of slab tear and partial underplating of the upper plate during the early stages of continental underthrusting. Discrete pieces may have later reattached to the lower plate at different times, partially exhumed, and then subducted to mantle-depths during the Scandian.&lt;/p&gt;

The volume conjugate in progressive metamorphism

&lt;p&gt;Volume changes during metamorphic reactions are key contributors to the physical changes of crystalline rocks. Assessing dehydration or hydration reactions in terms of conjugate &lt;em&gt;V&amp;#8211;T&lt;/em&gt; pseudosections provides indicators of transient departures in hydrostatic pressure and their impact on observed mineral equilibria. The expansion in volume of major dehydration events such as the breakdown of lawsonite or chlorite delineate zones of fluid overpressure that generate connectivity via fracturing. Net compressional reactions represent sinks for fluid consumption and the focussing of strain. The capacity of metamorphic rocks to generate or consume fluid along portions of the &lt;em&gt;P&amp;#8211;T&amp;#8211;V&lt;/em&gt; path exerts a fundamental control on the distribution of stresses in the crust and the observed mineral assemblages. Coupling a phase equilibria approach to mechanical modelling provides a quantitative framework to assess these changes in fluid pressure that can be compared to prominent case studies in rocks from New Caledonia and New Zealand.&lt;/p&gt;

Epidote – lawsonite coexistence in blueschist-facies block from the Tavşanlı Zone – Turkey: petrological implications

Metamorphic evolution of an epidote–lawsonite blueschist sample characterized by the coexistence of lawsonite and epidote from Sivrihisar area (Tavşanlı Zone) was studied herein in terms of petrology and mineral equilibria. Based on the textural evidence and phase composition, 2 prograde stages, defined by assemblage-I and -II, and 1 retrograde stage were recognized. Assemblage-I indicates epidote-blueschist facies conditions (12 ± 1 kbar / 485 ± 10 °C). Assemblage-II is characterized by the coexistence of epidote and lawsonite (17 ± 1 kbar / 515 ± 10 °C) corresponding to the interface of lawsonite blueschist and epidote blueschist facies. Phase diagram calculations and mineral compositions revealed that along this interface, an equilibrium field with lawsonite and epidote is stable. This closed-equilibrium field is controlled by high aH2O and an elevated Fe3+/Al ratio of minerals. Pressure-temperature (P–T) estimates and textural observations indicated a counter-clockwise path during the subduction and exhumation history. The preservation of lawsonite and epidote during the retrograde stage pointed to the fact that the path followed the stability field of lawsonite and epidote during exhumation.