Wet Granite Solidus Research Articles

Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.

High-pressure eclogite facies metamorphism and decompression melting recorded in paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)

Thermobarometry and monazite U-Th-Pb geochronology of the phengite + chloritoid + biotite + garnet + kyanite bearing mica-quartz schist and of the nearby phengite + garnet bearing Córrego do Peixoto granite from Itaguara Sequence (IS) of the southern São Francisco Craton (SFC) have revealed that, at 2.05±0.05 Ga, metamorphic peak achieved ∼18.5 kbar and ∼626 °C. These pressure-temperature (P-T) conditions correspond to amphibole eclogite facies metamorphism that occurred around 60 km depth, consistent with geothermal gradient of about 10 °C/km, typical of subduction zones in continental collision settings. The continental collision-related tectonic exhumation would have triggered a decompression process of the high-pressure (HP) mica-quartz schist, which was partially melted to generate the Córrego do Peixoto granite at 2.05 Ga in P-T conditions inserted in the wet granite solidus field. The younger 1.93±0.02 Ga monazite age found in both mica-quartz schist and granite could be related to the orogenic collapse-related decompression late stage with amphibolite facies overprint. The mica-quartz schist has been modified by this late stage metamorphism, which partially overprinted primary mineral assemblages. Taking into account that ophiolites, represented in the IS by metamafic-ultramafic rocks, are typically associated to an accretionary wedge in continental collision settings, it is here suggested that mica-quartz schist is part of a probable Paleoproterozoic accretionary wedge formed above a paleo-subduction zone. It would represent a unique case of a very old accretion prism linked to a continental collision.

Formation of magmatic brine lenses via focussed fluid-flow beneath volcanoes

Many active or dormant volcanoes show regions of high electrical conductivity at depths of a few kilometres beneath the edifice. We explore the possibility that these regions represent lenses of high-salinity brine separated from a single-phase magmatic fluid containing H2O and NaCl. Since chloride-bearing fluids are highly conductive and have an exceptional capacity to transport metals, these regions can be an indication of an active hydrothermal ore-formation beneath volcanoes. To investigate this possibility we have performed hydrodynamic simulations of magma degassing into permeable rock. In our models the magma source is located at 7 km depth and the fluid salinity approximates that expected for fluids released from typical arc magmas. Our model differs from previous models of a similar process because it is (a) axisymmetric and (b) includes a static high-permeability pathway that links the magma source to the surface. This pathway simulates the presence of a volcanic conduit and/or plexus of feeder dykes that are typical of most volcanic systems. The presence of the conduit leads to a number of important hydrodynamic consequences, not observed in previous models. Importantly, we show that an annular brine lens capped by crystallised halite is likely to form above an actively degassing sub-volcanic magma body and can persist for more than 250 kyr after degassing ceases. Parametric analysis shows that brine lenses are more prevalent when the fluid is released at temperatures above the wet granite solidus, when magmatic fluid salinity is high, and when the high-permeability pathway is narrow. The calculated depth, form and electrical conductivity of our modelled system shares many features with published magnetotelluric images of volcano subsurfaces. The formation and persistence of sub-volcanic brine lenses has implications for geothermal systems and hydrothermal ore formation, although these features are not explored in the presented model.

U-Pb ages and trace elements in metamorphic zircon and titanite from UHP eclogite in the Dabie orogen: constraints on P-T-t path

Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh-pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction-zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti-in-zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction-zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti-in-zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr-in-titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr-in-titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction-zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid-T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.

The Early Palaeozoic high-grade metamorphism at the active continental margin of West Gondwana in the Andes (NW Argentina/N Chile)

The evolution of the Early Palaeozoic orogen of West Gondwana in the Cambrian to Ordovician basement of the Andes between ~18° and 32° S is investigated for pressure and temperature conditions and age of metamorphism. It is characterized by mid-crust temperatures commonly above the wet granite solidus (~650°C). Widespread felsic migmatite and rare granulite formed at pressures of ca 0.5–0.7 GPa, locally 1.0 GPa. These rocks represent the deepest exhumed sections of the Early Palaeozoic crust. High pressure–low temperature rocks are absent. The crystallization ages, compiled from the literature in combination with new data, for near peak metamorphic conditions of these high-grade metamorphic rocks in NW Argentina and N Chile are ~530–500 Ma and ~470–420 Ma. Both age groups are spatially overlapping. Radiogenic isotope signatures (Sr, Nd, Pb) are used to characterize the Early Palaeozoic basement. The Pb and Sr isotope compositions of the Early Palaeozoic basement indicate mixing arrays between pre-Palaeozoic unradiogenic and radiogenic crust. Crustal residence ages (Sm–Nd TDM) indicate a prominent event of crust formation around ~2 Ga, which is known continent-wide. This material was recycled during Midproterozoic and Early Palaeozoic orogenies without prominent additions of new crust present in the isotope record, i.e. accretion of compositional exotic material is absent.

On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere

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

Temperature spectra of zircon crystallization in plutonic rocks

The Ti-in-zircon thermometer is a potentially powerful new petrological tool, but conclusions drawn from such data are meaningful only to the extent that the underlying interpretational framework is sound. In the case of interpretation of detrital zircon crystallization temperature spectra, it has been assumed that comparisons can be made with potential host rocks based on calculated bulk zircon saturation thermometry. We show by calculation that most igneous rocks formed at high temperature (>750 °C) will yield Ti-in-zircon temperatures (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathit{T}_{Ti}^{zir}\) \end{document}) well above the wet granite solidus. This prediction is borne out by results from the Dala igneous complex, southeastern Tibet, which show a 300 °C range in \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathit{T}_{Ti}^{zir}\) \end{document} beginning >100 °C above the calculated bulk zircon saturation temperature. Thus the dominant \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathit{T}_{Ti}^{zir}\) \end{document} peak of >4 Ga zircons at ∼680 °C implies a wet, anatectic magma source rather than an origin in intermediate and mafic magmas. Given that preservation and analytical effects select against, or obscure recognition of, zircons formed at low temperature, the best explanation for the low Hadean \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathit{T}_{Ti}^{zir}\) \end{document} peak is that it reflects derivation from rocks that experienced prograde melting conditions under near-water-saturated conditions.

The timing of high-temperature retrogression in the Reynolds Range, central Australia: constraints from garnet and epidote Pb-Pb dating

Lower Calcsilicate Unit metasediments and underlying migmatitic Napperby Gneiss metagranite at Conical Hill in the Reynolds Range, central Australia, underwent regional high-grade (∼680 to 720 °C), low-pressure/high-temperature metamorphism at 1594 ± 6 Ma. The Lower Calcsilicate Unit is extensively quartz veined and epidotised, and discordant grandite garnet + epidote quartz veins may be traced over tens of metres depth into pegmatites that pooled at the Lower Calcsilicate Unit-Napperby Gneiss contact. The quartz veins were probably precipitated by water-rich fluids that exsolved from partial melts derived from the Napperby Gneiss during cooling from the peak of regional metamorphism to the wet granite solidus. Pb stepwise leaching (PbSL) on garnet from three discordant quartz veins yielded comparable single mineral isochrons of 1566 ± 32 Ma, 1576 ± 3 Ma and 1577 ± 5 Ma, which are interpreted as the age of garnet growth in the veins. These dates are in good agreement with previous Sensitive High Resolution Ion Microprobe (SHRIMP) ages of zircon and monazite formed during high-temperature retrogression (1586 ± 5 to 1568 ± 4 Ma) elsewhere in the Reynolds Range. The relatively small age difference between peak metamorphism and retrograde veining suggests that partial melting and melt crystallisation controlled fluid recycling in the high-grade rocks. However, PbSL experiments on epidote intergrown with, and partially replacing, garnet in two of the veins yielded isochrons of 1454 ± 34 and 1469 ± 26 Ma. The ∼100–120 Ma age difference between intergrown garnet and late epidote from the same vein suggests that the vein systems may have experienced multiple episodes of fluid flow.

Measuring the seismic properties of Tibetan bright spots: Evidence for free aqueous fluids in the Tibetan middle crust

Seismic bright spots are commonly interpreted to mark fluid concentrations, but their nature (melt or aqueous) is usually inferred only from circumstantial evidence of the geologic setting. A band of bright spot reflections has been imaged by Project INDEPTH (International Deep Profiling of Tibet and the Himalayas) at about 15 km depth along 150 km of the northern Yadong‐Gulu rift, southern Tibet. We use INDEPTH three‐component wide‐angle seismic data to measure seismic velocities at the bright spot reflector, and theoretical rock physics bounds to constrain the nature of the fluids. Merging of data from multiple bright spots allows us to use a one‐dimensional approximation. Travel time modeling yields average P and S velocities for the upper crust above the bright spots of 5.3±0.2 and 3.2±0.2 km s−1, respectively. Reflection‐amplitude variation with offset (AVO) modeling constrains the P and S velocities of the bright spots to 3.0±0.8 and 1.6±0.8 km s−1, respectively. Multiple modeling procedures suggest these velocities are not model dependent. Our results imply that of the order of 10% volume of free aqueous fluids in the Tibetan middle crust produces the observed bright spot reflections. The presence of relatively large quantities of free aqueous fluids, presumably mostly saline supercritical H2O, does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus (about 650°C) and thus the maximum surface heat flow to ≤110 mW m−2. The observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern Tibetan crust.

Fluids, P– T paths and the fates of anatectic melts in the Earth's crust

This paper presents a model for the longevity and physical behaviour of anatectic melts in the Earth's crust. It is assumed that a thermal anomaly has caused partial fusion of biotite–plagioclase–quartz rocks, to produce a granitic melt and restitic solids. The focus is on the potential chemical and physical outcomes, as a function of the various P– T-fluid conditions. Rocks undergoing fluid-present melting will sustain an increase in bulk density and will become more gravitationally stable. Diapiric up-welling of such material should not occur. In fluid-absent melting, the bulk density will decrease and foster gravitational instability. Stresses in the surrounding rocks could, however, lead to fracture and rapid evacuation of the melt. With excess fluid and without melt segregation, isobaric cooling will cause immediate freezing. If T rises above the biotite–quartz melting reaction, freezing will still occur at the original reaction T. Diapiric up-welling should not occur. Biotite-bearing diatexites might be formed by large degrees of fusion, but magma ascent is unlikely. In isothermal decompression, immediate freezing will ensue, and the migmatite should contain orthopyroxene rather than biotite. With excess fluid and melt segregation, cooling will induce freezing at the fluid-saturated granite solidus. Magma ascent could, however, occur, even for X Fl H 2O =1. In isothermal decompression, melt will persist to the granite solidus for the imposed X Fl H 2O . Limited magma ascent could occur. All granites should contain primary igneous biotite. For fluid-deficient rocks without melt segregation, freezing will occur at the T of the original melting. If the reaction is overstepped, solidification will occur at the original X Fl H 2O . Magma formation is unlikely. Migmatites should contain either pyroxene retrogressed to biotite, or biotite alone. On decompression, immediate freezing is likely if T remains at the biotite–quartz reaction. For higher T, melt will survive decompression and freeze at the fluid-saturated solidus for some increased X Fl H 2O . Magma ascent is possible, and the melt will freeze very close to the wet granite solidus. In fluid-deficient rocks with melt segregation, cooling and crystallization will cause fluid saturation. In situ segregated melt will freeze at the original X Fl H 2O . Melt ascent will, however, cause exsolution of a CO 2-rich fluid. With fluid escape, melt will freeze close to the wet granite solidus. An increase in melt proportion may accompany magma ascent or source up-welling. Any low- P granite intrusion should be charnockitic. For fluid-absent rocks without melt segregation, freezing will occur on cooling to the T of the original reaction. Only transitory migmatites could be formed, but melt will survive decompression almost to the H 2O-saturated granite solidus. The rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite. With isothermal decompression, further melting may occur, forming an orthopyroxene-bearing granulite-facies migmatite. In fluid-absent situations with melt segregation, magma could be efficiently transported to higher crustal levels. If the melt proportion were insufficient for magma formation, granulite-facies migmatites would result. Ascending melt would progressively dissolve any entrained restite. The granites produced should be charnockitic. To aid in interpretation of the modelling results, we suggest that it may be slightly more common for the highest-grade metamorphic terranes to exhibit isobaric cooling paths, that fluid-present anatexis is normally limited to the shallower parts of the crust, and that fluid-absent reactions are more likely at greater depths. We conclude that the best models for natural fluid-present anatexis are those dealing with essentially pure H 2O fluid and that fluid-deficient conditions will dominate over fluid-excess. We also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements.

Melting, carbonic fluids and water recycling in the deep crust: an example from the Limpopo Belt, South Africa

The proposed retrograde orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt separates hydrated, amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma. Widespread rehydration of metapelitic granulites under conditions of 660–600 °C and ≥0.6 GPa, and CO2‐dominated fluid‐inclusion populations appear to suggest thorough flushing of the high‐grade crust with an externally derived carbonic fluid. However, past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium’ fluid in the evolution of the rocks. This contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes, generated by in situ fluid‐absent biotite melting along the prograde path. Model equilibrium fluid compositions suggest that reaction between this melt‐derived H2O and biogenic graphite produced CO2‐rich fluid compositions and potentially high fluid:rock ratios at the wet granite solidus. Declining temperature resulted in fluid compositions shifting to higher XH2O, with the precipitation of graphite essentially at the sites of initial fluid generation, thereby preserving original (pre‐metamorphic) isotopic heterogeneities. The hydration pattern of the Southern Marginal Zone appears to be a function of melt migration. In the hydrated zone, leucosomes generally approximate minimum melt compositions and in this zone H2O was effectively recycled between the prograde and retrograde assemblages. In contrast, leucosomes in the granulite grade portion of the terrane have lost a K2O‐ and H2O‐rich melt fraction, and although some hydration has occurred in this zone, orthopyroxene is generally preserved in metapelites. In a general context, in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid‐rock ratios at temperatures close to the wet granite solidus. This single process holds the potential for widespread retrogression of formerly high‐grade assemblages, at a variety of aH2O values, without external fluid input.

Chemical characteristics of migmatites: accessory phase distribution and evidence for fast melt segregation rates

Equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration (armouring, slow dissolution). When segregation occurs by channelised migration along high-porosity pathways, melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates. Evidence for channelised flow and deformation-enhanced melt segregation into boudin necks, fractures and micro-shears at low melt fractions is present in the Moine Kirtomy Migmatitie Suite (KMS) in Sutherland, Scotland. Melt migration distances are on a metre to tens of metres scale. Concordant leucosomes in stromatic migmatities in the KMS have low Zr contents, low LREE (light rare-earth element) and H (heavy) REE contents and positive Eu anomalies. REE patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of LREE- and HREE-bearing accessory phases in water-undersaturated melts. Melting in the KMS, however, occurred at or near the wet granite solidus, leaving biotite as a residual phase. Detailed back-scattered electron imaging shows that REE-bearing accessory phases remained as residual phases, and were concentrated in the melanosome and at the melanosome-leucosome boundary. Irregularly shaped patches of diatexite contain a small proportion of excess Zr, consistent with entrainment of melanosome-schlieren enriched in zircon. These data indicate that deformation-enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time-scale less than that required to saturate the melt in Zr. Leucosomes were thus prevented from equilibrating with accessory phases before extraction.

Thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada Batholith, Tehachapi Mountains, California

We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115–100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al contents of hornblende in granitoids imply igneous emplacement at ∼8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3–7 kbar) to the north. Metamorphic pressures and temperatures for garnet‐bearing paragneisses and metaigneous rocks were estimated on the basis of garnet‐hornblende‐plagioclase‐quartz and garnet‐biotite‐plagioclase‐quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite‐based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1–9.4 and 3.6–4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The ∼4‐kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet‐hornblende and garnet‐biotite temperatures are less coherent, likely owing to retrograde Fe‐Mg exchange, and range from 570° to 790°C, The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at “peak” metamorphic conditions near that of the wet granite solidus (>690°C). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet‐bearing assemblages suggest variable, mostly CO2‐rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of ∼30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after ∼15 km of uplift before ∼87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure‐temperature and uplift history is similar to those of high‐pressure/high‐temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan‐anted. When compared with higher‐level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz‐rich at these levels, suggestive of a weak, ductile middle crust susceptible to prolonged deformation and possible delamination.