Rare earth and other trace element mobility during mylonitization: a comparison of the Brevard and Hope Valley shear zones in the Appalachian Mountains, USA

In progressing from a granitoid mylonite to an ultramylonite in the Brevard shear zone in North Carolina, Ca and LOI (H₂O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al–Ti–Fe isocon. The increase in Ca and LOI is related to a large increase in retrograde epidote and muscovite in the ultramylonite, the decreases in K, Na, Si, Ba and Sr reflect the destruction of feldspars, and the decrease in Mg is related to the destruction of biotite during mylonitization. In an amphibolite facies fault zone separating grey and pink granitic gneisses in the Hope Valley shear zone in New England, compositional similarity suggests the ultramylonite is composed chiefly of the pink gneisses. Utilizing an Al–Ti–Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M-HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Ba are enriched, and Ca, Na, P, Sr and LREE are lost during deformation. In contrast to the Brevard mylonite, the Hope Valley mylonite appears to have increased in volume by about 70%, chiefly in response to an introduction of quartz. Chondrite-normalized REE patterns of granitoids from both shear zones are LREE-enriched and have prominent negative Eu anomalies. Although REE increase in abundance in the Brevard ultramylonites (reflecting the volume loss), the shape of the REE pattern remains unchanged. In contrast, REE and especially LREE decrease in abundance with increasing deformation of the Hope Valley gneisses. Mass balance calculations indicate that ≥95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15–40% of the REE can be accounted for collectively by titanite, apatite and zircon. Possible sites for the remaining REE are allanite, fluorite or grain boundaries. Loss of LREE from the pink gneisses during deformation may have resulted from decreases in allanite and perhaps apatite or by leaching ofy REE from grain boundaries by fluids moving through the shear zone. Among the element ratios most resistant to change during mylonitization in the Brevard shear zone are La/Yb, Eu/Eu*, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U and Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu*, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and Hf/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoliths of deformed rocks.     

Localization control for chlorite breccia deformation beneath Catalina detachment fault,Rincon Mountains,Tucson, Arizona

The structural location of an approximately 3-km stretch of the Catalina detachment fault zone appears to have been controlled by an ultramylonite shear zone within mylonites of the Catalina–Rincon metamorphic core complex. The Catalina detachment fault zone consists of the detachment fault surface, a 3–5 m interval of cataclasite and ultracataclasite, up to ∼50 m of chlorite breccia, and a discrete subdetachment fault. Beneath the subdetachment fault is a km-scale thickness of mylonites. Progressive ductile-to-brittle shear-zone evolution of the fault-rock stratigraphy started with mylonitization, was followed by overprinting of mylonites by chlorite brecciation, and culminated in the formation of the Catalina detachment fault and associated ultracataclasites. The detachment fault is cospatial with and subparallel to the zone of chlorite breccia. The subdetachment fault is subparallel to the interval of chlorite brecciation and to the detachment fault. The ‘plane’ of projection of the approximately 30-m thick ultramylonite shear zone within the mountain of mylonite, when followed downdip, coincides with the base of the ‘chlorite breccia’ brittle shear zone. Ultramylonite is preserved in places in the immediate lower plate of the subdetachment fault. The position and orientation of the subdetachment fault appears to have been controlled by an ultramylonite shear zone within the lower-plate mylonites. The rheological properties and orientation of this ultramylonite shear zone favored its reactivation as the brittle sole fault of the zone of chloritic brecciation.     

韧性剪切变形对岩石地球化学行为的制约——以北阿尔金巴什考供韧性剪切带为例

韧性剪切变形对岩石地球化学行为的制约一直是地质学家们探讨的课题。本文以构成北阿尔金红柳沟——拉配泉俯冲碰撞杂岩带与北阿尔金地块边界的巴什考供斜向逆冲型韧性剪切带为例，通过对韧性剪切带内花岗岩变形前后不同变形强度构造岩的地球化学组成进行对比，确定等比线斜率，探讨韧性变形对岩石体积和成分变异的影响。计算结果表明，在糜棱岩化过程中，糜棱岩化花岗岩体积亏损21％，花岗质糜棱岩体积亏损31％。质量平衡计算结果和等比线图表明，韧；陛剪切作用导致SiO2，流失量最大，A12O3、K2O及Ba、Rb、Sr等都有不同程度的丢失，显示出较强的活动性，MnO、P2O5、Sc位于等比线上或附近，表现出相对的稳定性。岩石中活动组分的变异是流体渗滤作用引起的，不活动组分的变异是体．积亏损造成的。     

Generation of pseudotachylite under granulite facies conditions, and its preservation during cooling

Abstract Discontinuous ultramylonite zones cut Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, and preserve evidence of ductile non-coaxial flow and reverse sense of shear. Cross-cutting relationships indicate that ultramylonite deformation involved overthrusting to the east, but progressively rotated to involve overthrusting to the north; rotation of the principal compressive stress axes is inferred. Extensive pseudotachylite developed during ultramylonitization, the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation. Neoblastic sillimanite indicates ultramylonitization occurred at >520° C. On the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at >700° C, and ≤7.3 ± 0.5 kbar, at a_H2O± 0.3 and low a_CO2. Comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the Rayner Complex experienced a late increase in pressure of 1–2 kbar during ultramylonitization. The P-T-a_H2O conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting, pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates. Most of the pseudotachylite veins are undeformed; the mechanism(s) of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite, further strain having been mostly accommodated by a new or subsidiary shear zone. Renewed stress at reduced strain rates, or renewed stress in zones in which the proportion of pseudotachylite was significantly higher, could have led to the rare occurrences of deformed pseudotachylite. The preservation of fine-grained pseudotachylite is dependent on it remaining dry.     

Marble mylonites in the Bancroft shear zone, Ontario, Canada: microstructures and deformation mechanisms

Mylonitization of medium-grade marbles in the Bancroft shear zone, Ontario, Canada, is characterized by decreasing grain-size of both calcite and graphite, and a variety of textures. Calcite grain-sizes vary from several millimeters in the protolith, to 50–200 μm in mylonite, to <30 μm in ultramylonite. Corresponding calcite grain shapes are equant in the protolith, elongate in protomylonite (first-developed dimensional preferred orientation), equant in coarse mylonite, elongate in fine mylonite (second-developed dimensional preferred orientation) and generally equant in ultramylonite, which suggests that external energy (applied stress) that tends to elongate grains competed with internal energy sources (e.g. distortional strain) that favor equant shapes. Graphite grain-size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite. In the mylonitic stages, graphite is present as dark bands, while in the ultramylonitic stage it is preserved as a fine coating on calcite grains.Based on textural evidence, twinning (exponential creep; regime I), dynamic recrystallization (power law creep; regime II) and possibly grain boundary sliding superplasticity (regime III) are considered the dominant deformation mechanisms with increasing intensity of mylonitization; their activity is largely controlled by calcite grain-size. Calcite grain-size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization, as indicated by the occurrence of core and mantle or mortar structures, and by the grain-size of subgrains and recrystallized grains. Grain elongation in S-C structures indicates the activity of migration recrystallization; these structures are not the result of flattening of originally equant grains. Differential stress estimates in coarse mylonites and ultramylonites, based on recrystallized grain-size, are 2–5 and 14–38 MPa, respectively. Initial grain-size reduction of graphite occurred by progressive separation along basal planes, analogous to mica fish formation in quartzo-feldspathic mylonites.Calcite-graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 ± 50°C, which, combined with a differential stress value of 26 MPa, gives a strain rate of 1.2 x 10⁻¹⁰s⁻¹ based on constitutive equations; corresponding displacement rates are <38 mmyr⁻¹.     

Rock Deformation, Component Migration and ^18O/^16O Variations during Mylonitization in the Southern Tan-Lu Fault Belt

This paper discusses the relationship between the volume loss, fluid flow and component variations in the ductile shear zone of the southern Tan-Lu fault belt. The results show that there is a large amount of fluids flowing through the shear zone during mylonitization, accompanied with the loss of volume of rocks and variations of elements and oxygen isotopes. The calculated temperature for mylonitization in different mylonites ranges from 446 to 484℃, corresponding to that of 475 to 500℃ for the wall rocks. The condition of differential stress during mylonization has been obtained between 99 and 210 MPa, whereas the differential stress in the wall rock gneiss is 70-78 MPa. The mylonites are enriched by factors of 1.32-1.87 in elements such as TiO2, P2O5, MnO, Y, Zr and V and depleted in SiO2, Na2O, K2O, Al203, Sr, Rb and light REEs compared to their protolith gneiss. The immobile element enrichments are attributed to enrichments in residual phases such as ilmentite, zircon, apatite and epidote in mylonites and are interpreted as due to volume losses from 15% to 60% in the ductile shear zone. The largest amount of SiO2 loss is 35.76 g/100 g in the ductile shear zone, which shows the fluid infiltration. Modeling calculated results of the fluid/rock ratio for the ductile shear zone range from 196 to 1192 by assuming different degrees of fluid saturation. Oxygen isotope changes of quartz and feldspar and the calculated fluid are corresponding to the variations of differential flow stress in the ductile shear zone. With increasing differential flow stress, the mylonites show a slight decrease of δ^18O in quartz, K-feldspar and fluid.     

Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural and textural fabrics and their kinematic significance

The deformation history of a monophase calcite marble shear zone complex on Thassos Island, Northern Greece, is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system (shear zone boundary, SZB). Strain localization within the massive marble complex is linked to decreasing P–T conditions during the exhumation process of the metamorphic core complex. Solvus thermometry indicates that temperatures of 300–350°C prevailed during part of the shear zone deformation history. The coarse-grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation. A component of heterogeneous non-coaxial deformation is first recorded within the adjacent protomylonite. Enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone, where high strain was accommodated by non-coaxial flow. This study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations (single c-axis maximum perpendicular to the SZB) by different dominant deformation mechanisms. Separated a-axis pole figures (+a- and −a-axis) show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry.     

Carbon Enrichment in the Lithospheric Mantle: Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

It is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth’s interior and the reasons for carbon enrichment remain unclear. In this study, we report CO₂-rich olivine-hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present ...     

Transpressional granite-emplacement model: Structural and magnetic study of the Pan-African Bandja granitic pluton (West Cameroon)

The Pan-African NE–SW elongated Bandja granitic pluton, located at the western part of the Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE–SW and NNE–SSW strike directions with moderate to strong dip respectively in its northern and central parts. This mostly, ferromagnetic granite displays magnetic fabrics carried by magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33 and 35/59 respectively in its northern, central and southern parts and (ii) a subhorizontal magnetic lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and southern parts. Magnetic lineation shows an ‘S’ shape trend that allows to (1) consider the complete emplacement and deformation of the pluton during the Pan-African D ₂ and D ₃ events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons in the belt as: 686 ±17 Ma (Rb/Sr) for D ₁ age of metamorphism recorded in gneiss; and the period between 604–557 Ma for D ₂–D ₃ emplacement and deformation age of the granitic pluton in a dextral ENE–WSW shear movement.     

苏鲁地体南部南岗-高公岛韧性剪切带的变形记录、组分迁移和体积亏损

南岗-高公岛韧性剪切带是一条自南东向北西斜向逆冲的岩片边界性构造带,糜棱岩化作用强烈。韧性变形过程中,岩石中的主要矿物长石、黑云母等发生了不同程度的分解或蚀变并导致组分迁移。其中,活动组分的迁移是流体渗滤作用引起的,不活动组分的变异是体积亏损造成的。通过对韧性剪切带内不同变形程度岩石主要地球化学组分的对比分析,确立等比线斜率,计算出初糜棱岩的体积亏损率约为17%,糜棱岩的体积亏损率约为27%;质量平衡计算结果及等比线图表明韧性剪切作用导致SiO2流失量最大,其次为Al2O3,FeO、CaO、Na2O等都有不同程度的丢失,表明它们具有较强的活动性,MgO位于等比线上方,属于带入组分。糜棱岩中的石英或长英质条带和团块是长石的钠黝帘石化、绢云母化和黑云母的绿泥石化等导致SiO2、Al2O3、Na2O、CaO、FeO等活动组分从岩石中析出迁移形成的。这种岩石体积亏损和组分迁移是部分难溶组分富集的重要机制,对研究韧性剪切带中长英质条带和部分贵金属矿床的形成机制具有重要的指导作用。     

Structural and textural development in Singhbhum shear zone,eastern India

The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation, folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa during mylonitization.     

Grain-size-sensitive creep of plagioclase accompanied by solution–precipitation and mass transfer under mid-crustal conditions

We report here on a study of three deformed granitoids: two mylonites and an ultramylonite from the inner ductile shear zone of the Ryoke metamorphic belt, SW Japan. Monophase layers composed of quartz, plagioclase or K-feldspar are present in all samples. The plagioclase-rich layers consist of grains 6–10 μm in size, and sometimes include patchy K-feldspar and quartz, indicating solution-precipitation. In the mylonite, the fine-grained plagioclase is mainly An_23–25 and, the composition of plagioclase porphyroclast is An_21–39 without any significant maximum. The An compositions together with textural observations indicate that fine-grained plagioclase nucleated from solution with mass transfer during deformation. In the ultramylonite, fine-grained plagioclase is widely changed to be An_15–37, indicating that the grain-size-reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution–precipitation process, which results in the composition concentrated around An₃₀. In all samples, the crystallographic orientations of fine-grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts. Grain-size-sensitive creep occurred during rock deformation subsequent to the process of solution–precipitation that involved mass transfer via fluids.     

Transition from I‐type to A‐type magmatism in the Sanandaj–Sirjan Zone,NW Iran: an extensional intra‐continental arc

The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO₂ content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A‐type affinity with high Na₂O + K₂O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO₂ levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na₂O + K₂O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO₂ content (48.5%–55.9 wt.%) with metaluminous and calc‐alkaline compositions, relatively lower Na₂O + K₂O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geochemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic subduction of the Neo‐Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley & Sons, Ltd.     

Role of plagioclase and reaction softening in a metagranite shear zone at mid‐crustal conditions (Gotthard Massif,Swiss Central Alps)

A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre‐scale shows an evolution from a weakly deformed metagranite (Qtz_I–Kfs_I–Ab_I–Bt_I ± Pl_II–Zo_I–Phg_I–Grt) to a fine banded ultramylonite (Qtz_II–Kfs_II–Ab_II–Pl_II–Bt_II–Phg_II ± Grt–Zo_II). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K‐feldspar. The continuous recrystallization of plagioclase during deformation into a very fine‐grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed‐system conditions with the destabilization of metastable Ab_I–Zo_I porphyroclasts into fine‐grained (20–50 μm sized) Ab_II–Pl_II aggregates, and with minor crystallization of phengite at the expense of K‐feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H₂O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine‐grained Ab_II–Pl_II aggregate into a more Pl_II‐rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K‐feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one‐third of the bulk rock volume in granitic systems and forms a stress‐supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P–T conditions and/or bulk composition must be taken into account, together with quartz and K‐feldspar, in order to understand strain localization processes in granites.     

Petrogenesis of Kejie Granite in the Northern Changning-Menglian Zone, Western Yunnan: Constraints from Zircon U-Pb Geochronology, Geochemistry and Hf Isotope

The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80–77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2(71.68%–72.47%), K2O(4.73%–5.54%), total alkali(K2O + Na2O = 8.21%–8.53%), K2O/Na2O ratios(1.36–1.94) and low P2O5(0.13%–0.17%), with A/CNK of 1.025–1.055; enriched in U, Th, and K, depleted in Ba, Nb, Sr, Ti, P and Eu. They are highly fractionated, slightly peraluminous I-type granite. The two samples of the Kejie pluton give a large variation of εHf(t) values(-5.04 to 1.96) and Hf isotope crustal model ages of 1.16–1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock(801°C–823°C) show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.     

Late Pliocene lamproites from Bucak,Isparta (southwestern Turkey): Implications for mantle ‘wedge’ evolution during Africa-Anatolian plate convergence

Magmatism in the Kirka–Afyon–Isparta (KAI) region, southwestern Turkey, shows a temporal progression from calc-alkaline to ultrapotassic affinity. Magmatic activity is associated with the geodynamic evolution of the ‘Isparta Angle’ and is typical of a collision-affected convergent plate margin, most magmas being enriched in potassium and other large-ion lithophile elements (LILE) and depleted in high-field strength elements (HFSE) such as Ti, Zr, Nb, Ta, and Hf. However, Late Pliocene ultrapotassic lamproites in the south of ‘Isparta Angle’ show HFSE-rich incompatible element distributions, similar to those of ‘non-orogenic’ intraplate leucite basalts (ILB) and oceanic island basalts (OIB). Their association with HFSE-depleted ‘orogenic’ magmas suggests that ultrapotassic character reflects primarily crustal contamination of their mantle sources, rather than magma-wallrock reaction effects. Their relatively high content of Fe and Ti (for equivalent Mg content), and SiO₂-undersaturated character suggest that they segregated at relatively high pressures (>ca. 2.0 GPa) from fertile sources. In contrast, the older SiO₂-saturated, Afyon (orogenic) magmas which, for equivalent Mg content, show lower contents of Fe and Ti, are better explained as partial melts segregating at ca. 1.0–1.5 GPa from refractory (basalt-depleted) sources, similar to those of basalt-borne xenoliths tapping the lithospheric mantle. The notion of variably fertile contaminated mantle sources is compelling, but needs to be constrained in terms of the dynamic interaction between the lithosphere and asthenosphere and their respective contamination histories. Given the unlikelihood of in situ partial melting of the continental lithosphere mantle, we propose that both orogenic and non-orogenic magmas are generated at different pressures from sources within the convecting asthenosphere, contaminated by both lithospheric mantle and crustal components. This model rests on two testable conjectures: firstly, the interpretation that the continental lithospheric mantle is residual from partial melting at an earlier stage of its history and, secondly, that such material is incorporated into the asthenospheric flow field during and following subduction. The first of these is supported by the ambient compositions of continental basalt-borne xenoliths, while the second is contingent on the prediction that lithospheric mantle may be rheologically transformed during subduction-related hydration prior to its incorporation. The proximity of the Bucak lamproites to the Menderes Massif, a suspected Archean cratonic fragment, highlights the resemblance of these unusual rocks to intra-plate leucite-bearing lamproites elsewhere, whose genesis has been linked to mantle ‘wedge convection’ triggered beneath cratonic and circumcratonic lithosphere domain boundaries.     

Element residence and transport during subduction-zone metasomatism: evidence from a jadeitite-serpentinite contact,Guatemala

Jadeitite is a rare constituent of serpentinite-matrix mélange bodies from certain subduction complexes. Most jadeitite crystallizes from Na-, Al-, and Si-bearing fluids that are apparently derived from multiple subduction-zone sources. Even though jadeitite is near-end-member NaAlSi₂O₆ in major element composition and is volumetrically minor in subduction complexes, its trace elements and stable isotopes appear to record fluid compositions not directly seen in other subduction zone metasomatic systems.

Prior to our work, how jadeitite-forming fluids interact with serpentinite host rocks and serpentinizing fluids were largely unknown, because serpentinite-to-jadeitite contacts are generally not exposed. In the Sierra de las Minas, Guatemala, we have studied a 3 m-wide pit transecting the contact between a mined-out jadeitite body and its host serpentinite. An apparent transition zone between the former jadeitite and nearby serpentinite exposed in the mine pit contains four texturally distinct rock types of differing outcrop colours, composed of albitites and meta-ultramafic rocks. (The jadeitite body is now represented only by a large spoil pile.) Seven samples from the contact zone, jadeitite from the spoil pile, a serpentinite outcrop approximately 1 m outside the pit, and a jadeitite nodule within the contact zone albitite were analysed for major, minor, and trace elements.

Abundances of Al₂O₃, Na₂O, MgO, FeO, Cr, Ni, and Sc track the contact between sheared albitite and foliated meta-ultramafic rocks. These elements change from values typical of Guatemalan jadeitites in the jadeitite block and albitites in the contact zone to values for Guatemalan meta-ultramafic rocks and serpentinites across the contact zone. In addition, the abundances of SiO₂, CaO, Fe₂O₃, K₂O, Rb, Cs, and Y show important features. Of greatest interest, perhaps, approximately 15 cm from the contact with meta-ultramafic rock, Zr, U, Hf, Pb, Ba, Sr, Y, and Cs in albitite are greatly enriched compared to elsewhere in the contact zone. Element enrichments spatially coincide with the appearance, increase in modal abundance, and/or increase in grain sizes of zircon, rare earth element (REE) rich epidote, titantite, and celsian within albitite. All of these ‘trace-element-rich’ accessory minerals show poikiloblastic inclusions of albite, which suggests that they grew concomitantly in the metasomatic zone.

Graphical and computational methods of evaluating mass changes of metasomatites relative to likely protoliths show that, near the contact, fewer minor and trace elements in albitite show 1:1 coordination with presumed protoliths. Most metasomatitites are enriched in large-ion lithophile elements (LILE) and heat-producing elements (HPE) relative to likely protoliths. Albitite near the contact with meta-ultramafic rocks also shows ultramafic components. Except for a Ca-rich actinolite schist zone, the meta-ultramafic rocks are depleted in LILE and HPE relative to serpentinite; host serpentinite is itself under-abundant in these elements relative to average upper mantle or chondrite.

In summary, the metasomatic zone shows more evidence for the introduction of components to albitite and actinolitic meta-ultramafic rock than it does for exchange of protolith components between jadeitite and serpentinite. The fluid that presumably formed the metasomatites was sufficiently rich in LILE and high-field-strength elements (HFSE) to both saturate and grow minerals in which Zr, Ba, and Ti are essential structural constituents and/or HFSE, LILE, and HPE minor to moderate substituents. These geochemically diverse element groups were fixed in albitite via the crystallization and growth of new accessory minerals within these rocks during albititization. The amount of LILE and HPE-depleted meta-ultramafic rock appears to be too small to call upon a local source for the LILE and HPE-enrichment seen in albitites. Therefore, LILE and HPE must be of exotic origin, carried and deposited by fluids within the albitites at the jadeitite-serpentinite contact. This contact clearly testifies to an alteration style that involved crystallization of ‘trace-element’-rich minerals during fluid flow; this process appears to be essential to mass transfer within subduction zones.     

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H⁺ and/or H₂O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.     

Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D₁) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D₁ migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D₁ fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.     

Orthomagmatic quartz and post-magmatic carbonate veins in a reported porphyry copper deposit,Andean Intrusive Suite,Livingston Island,South Shetland Islands

A previously reported porphyry Cu + Mo deposit in an Eocene pluton within the South Shetland Island magmatic arc has been re-interpreted as three distinct hydrothermal assemblages. The oldest assemblage (1) exsolved under confinement from the deep (～6 km?) cooling magma whereas assemblages (2) and (3) formed during tectonic ± magmatic episodes at depths of < 1.5 km in the late Cenozoic. The three assemblages occur over the 5 × 11 km mapped in Barnard Point tonalite pluton. Assemblage (1) comprises shallowly dipping sheets of aplite, biotite + tourmaline pegmatite, massive ‘grey’ quartz, and quartz + tourmaline + bornite + chalcopyrite + molybdenite veins. Magnetite + tourmaline + chalcopyrite breccias have associated biotite, K-feldspar and muscovite alteration. Fluid inclusions indicate formation from hot (～600°C), saline (40 equivalent weight % NaCl + CaCl₂) aqueous-carbonic fluids that exsolved from the partly consolidated magma. The primary control on solution chemistry and nature of fracturing was the depth of pluton emplacement. Assemblage (2) consists of steep, vuggy veins and country-rock breccias, with thick propylitic alteration selvages, cemented by microcrystalline quartz, complex inter-growths of FeMg carbonate, bladed barite and trace amounts of bornite and chalcopyrite. These rocks, previously described as breccia (sensu ‘pebble’) dykes in the porphyry complex, are reinterpreted as an influx of moderately hot (175–330°C), weak to moderately saline (2–21 EWP NaCl), aqueous-carbonic fluids that underwent isobaric boiling at 0.8 to 1.3 km depth. Assemblage (3) consists of thin, hematitic fault infillings formed during a second episode of brittle faulting.