Physical Conditions Of Metamorphism Research Articles

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

ABSTRACTThe Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.

Conductively driven, high‐thermal gradient metamorphism in the Anmatjira Range, Arunta region, central Australia

AbstractLA–ICP–MS in situ U–Pb monazite geochronology and P–T pseudosections are combined to evaluate the timing and physical conditions of metamorphism in the SE Anmatjira Range in the Aileron Province, central Australia. All samples show age peaks at c. 1580–1555 Ma, with three of five samples showing additional discrete age peaks between c. 1700 and 1630 Ma. P–T phase diagrams calculated for garnet–sillimanite–cordierite–K‐feldspar–ilmenite–melt bearing metapelitic rocks have overlapping peak mineral assemblage stability fields at ~870–920 °C and ~6.5–7.2 kbar. P–T modelling of a fine‐grained spinel–cordierite–garnet–biotite reaction microstructure suggests retrograde P–T conditions evolved down pressure and temperature to ~3–5.5 kbar and ~610–850 °C. The combined geochronological and P–T results indicate the SE Anmatjira Range underwent high‐temperature, low‐pressure metamorphism at c. 1580–1555 Ma, and followed an apparently clockwise retrograde path. The high apparent thermal gradient necessary to produce the estimated P–T conditions does not appear to reflect decompression of high‐P assemblages, nor is there syn‐metamorphic magmatism or structural evidence for extension. Similar to previous workers, we suggest the high‐thermal gradient P–T conditions could have been achieved by heating, largely driven by high heat production from older granites in the region.

Analysis of ordinary chondrites using powder X-ray diffraction: 2. Applications to ordinary chondrite parent-body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X-ray diffraction (XRD)-measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD-derived olivine and low-Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H2O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.

An overview of geological studies of JARE in the Napier Complex, Enderby Land, East Antarctica

Abstract Subsequent to the reconnaissance fieldwork in 1982, the Japanese Antarctic Research Expedition (JARE) carried out extensive geological studies that focused on structural and tectonic aspects, petrology, geochemistry and geochronology of the Napier Complex in Enderby Land, East Antarctica. Detailed field investigations in several key areas, including geological mapping of the Mt. Riiser-Larsen area and Tonagh Island, revealed that the Napier Complex comprises layered and massive gneiss units, of which the layered unit is composed of garnet felsic gneiss, orthopyroxene felsic gneiss, pelitic and basic gneisses, impure quartzite, and minor metamorphosed banded iron formation, whereas the massive unit consists mainly of orthopyroxene felsic gneiss. The boundary between the units is transitional in the Mt. Riiser-Larsen area, in which metamorphosed anorthosite and ultramafic rocks occur as thin layers, or blocks or pods, but on Tonagh Island the boundary is closely associated with the shear zone. Nine deformation episodes (D 1 –D 9 ) were suggested for Tonagh Island. These results of fieldwork were presented in detail in two geological maps. Geochemical studies showed that (1) garnet–sillimanite gneisses and garnet-rich felsic gneisses were derived from mudstone and sandstone, respectively, both enriched in MgO, Cr and Ni; (2) orthopyroxene felsic gneisses have a close REE affinity with Archaean tonalite–trondhjemite–granodiorite (TTG); (3) basic gneisses were derived from light rare earth element (LREE)-enriched or -depleted basalts; (4) meta-ultramafic rocks are comparable with komatiite and related depleted mantle peridotite. This suite of protoliths is reminiscent of Archaean greenstone–granite belts. Precise analyses of physical conditions of metamorphism were carried out by using reliable approaches such as feldspar thermometry, alumina content of orthopyroxene, inverted pigeonite and bulk-rock compositions, and clino- and orthopyroxene compositions with different textures (porphyroblastic and neoblastic), and the results suggested that the maximum metamorphic temperature might have reached 1130 °C (i.e. ultrahigh-temperature (UHT) metamorphism). P – T evolution of the Napier UHT metamorphism was examined by analyses of reaction textures combined with fluid inclusion studies, suggesting both clockwise (Bunt Island) and counterclockwise (Mt. Riiser-Larsen and Tonagh Island) P – T – t paths. U–Pb sensitive high-resolution ion microprobe and secondary ionization mass spectrometry zircon ages from the Mt. Riiser-Larsen area and Tonagh Island indicate three stages of protolith formation at around 3.28–3.23, 3.07 and 2.68–2.63 Ga, and two contrasting ages for the timing of peak UHT metamorphism at either c. 2.55 or c. 2.51–2.45 Ga. On the basis of these results, more comprehensive studies on the Napier Complex are essential in the future for understanding (1) the role and age of TTG protolith and (2) the origin and timing of UHT metamorphism.

Low‐grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, northern California

Abstract Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low‐grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H2O + FeMg‐1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5‐7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5‐10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.

Metamorphism in the Solitude Range, southwestern Rocky Mountains, British Columbia: comparison with adjacent Omineca Belt rocks and tectonometamorphic implications for the Purcell Thrust

Rocks of the Solitude Range, British Columbia, have been metamorphosed from chloritoid–chorite-zone to kyanite-zone conditions. The grade of metamorphism increases southwestward toward the Rocky Mountain Trench (RMT) and the Omineca Belt. Isograds crosscut lithologies and trend more northerly than deformation 2 (D2) structures and the RMT. They are thought to have been quenched syn- to post-D2. Pelitic (Mahto Formation) and calc-pelitic (Tsar Creek unit) rocks contain assemblages that reflect the increase in metamorphic grade. Physical conditions of metamorphism are estimated to be approximately 450–540 °C from the garnet to the kyanite zone; pressures averaged 6–7 kbar (1 kbar = 100 MPa). The pressures, temperatures, and metamorphic assemblages are very similar to those of the Adamant Range, which lies across the Purcell Thrust, to the southwest. This is in contrast with the Big Bend area, to the northwest, where differences in pressure across the Purcell Thrust (PT) have been documented. Two possible models to explain these contrasting relationships are presented. One model suggests that there was post-movement heating on the PT, which reduced the metamorphic contrast across the PT. The second model suggests that a combination of thrust and normal faulting, including warping of isobaric surfaces, has produced an apparently unbroken metamorphic sequence across the PT.

Petrology of Mg-Mn amphibole-bearing assemblages in manganese silicate rocks of the Sausar Group, India

AbstractMg-Mn amphibole (tirodite), with or without pyroxmangite in the total absence of pyroxenes and high-calcic pyroxenoids, occurs in the Mn silicate rocks of the Sausar Group, India. The rocks were metamorphosed to amphibolite facies condition (T ∼ 650°C, P ∼ 6 kbar). Tirodite-pyroxmangite pairs developed in both carbonate-free and rhodochrosite-bearing assemblages. Also tirodite coexists with either kutnahorite or manganoan calcite in the absence of pyroxmangite. Mineral reactions inferred from modal abundances and compositions of the phases indicate stabilization of the amphibole alone from a bivalent cation-bearing residual unbuffered XCO2 system with XMn < 0.3. On the other hand, tirodite-pyroxmangite pairs appeared in unbuffered low to intermediate XCO2 assemblages with XMn > 0.35. Pyroxenes and high-calcic pyroxenoids did not appear in the present situation, though they occur elsewhere in rocks with broadly similar contents of immobile components. Closely associated assemblages of diverse mineralogy suggest that the XMn and XCO2, rather than the physical conditions of metamorphism, are the decisive factors in promoting the observed phase assemblages.

Calderite-rich garnets from metamorphosed manganese silicate rocks of the Sausar Group, India, and their derivation

AbstractManganiferous garnets occur in metamorphosed Mn silicate-oxide and silicate-carbonate-oxide rocks of the Sausar Group, India. The garnets of the carbonatic rock show maximum calderite content (∼50 mole%). Increased Ca content in the bulk has been observed to be sympathetically related to the concentration of calderite, rather than the expected andradite component of the garnets. This is the consequence of the preferential partitioning of Ca in coexisting pyroxmangite and/or carbonate. Petrochemical characteristics of the diverse assemblages in response to the physical conditions of metamorphism show that the calderite solubility in manganiferous garnet is not only a function of pressure. Such characteristics only indicate that the mobile components in the bulk influenced the mineralogy of the assemblages at the ambient physical conditions of metamorphism, and this in turn controlled the calderite solubility in garnet.

Metamorphism of the Arseno Lake area, N.W.T., Canada: an Abukuma facies series of Aphebian age

Progressive mineralogical and mineral–chemical changes are described for metapelitic rocks from an Abukuma-type metamorphic series ranging from greenschist to upper amphibolite – granulite facies in the Bear Structural Province, Northwest Territories, Canada.The first appearance of the following minerals defines six isograds: biotite; andalusite; cordierite (muscovite + chlorite out); sillimanite (andalusite out); sillimanite + K-feldspar (muscovite + quartz out); and almandine + K-feldspar ± cordierite (biotite + sillimanite + quartz out).Electron microprobe analyses of the Fe–Mg silicates, biotite, cordierite, and garnet, display two distinct trends of mineral chemistry with increasing metamorphic grade. In the almandine + K-feldspar ± cordierite zone, where garnet is present, Fe/(Fe + Mg) decreases in all of the Fe–Mg silicates observed. However, in the cordierite zone and in the higher grade rocks where garnet is absent, Fe/(Fe + Mg) increases in both biotite and cordierite. Ilmenite and rutile are involved in all continuous reactions and lead to increasing Fe/Mg with grade unless garnet is a product of reaction. There is also a displacement towards lower Fe content at the sillimanite + K-feldspar isograd.The scale of equilibration decreases to 1–2 mm in the almandine + K-feldspar ± cordierite zone, which is most probably a function of the decrease of [Formula: see text] and therefore [Formula: see text]in the metamorphic fluid with increasing metamorphic grade.The physical conditions of metamorphism in the Arseno Lake area range from [Formula: see text] at 2–2.5 kbar with[Formula: see text] in the chlorite zone to ≥650 °C at 3.5–4.0 kbar where [Formula: see text] in the almandine + K-feldspar ± cordierite zone.

High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane, Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago. During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet—pyroxene charnockites indicates the presence of CO 2-rich intergranular fluids ( X CO 2 ⩾ 0.5) in these rocks during metamorphism. The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol—Spi), Fabriès (Ol—Spi), Wells (Opx—Cpx), Powell (Opx—Cpx), Ellis and Green (Gra—Cpx), Lal and Raith (Gra—Opx), and Danckwerth and Newton (Al 2O 3-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar—Opx/Cpx—Plag—Qtz). The P— T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton.

Regional geothermobarometry in the granulite facies terrane of South India

ABSTRACTIn the southern part of the Archaean craton of South India, an approximately 3.4–2.9 b.y. old migmatite–gneiss terrane (Peninsular gneiss complex) has been subjected to granulite facies metamorphism about 2.6 b.y. ago. During this event, the extensive charnockite-khondalite zone of southern India developed. A younger metamorphism (Proterozoic?) led to retrogression of the charnockites and khondalites, mainly under the conditions of the amphibolite facies.The physical conditions of metamorphism have been evaluated by applying methods of geothermobarometry to the widespread charnockitic assemblages with garnet, orthopyroxene, clinopyroxene, plagioclase, and quartz. The interpretation of the P–T estimates includes a critical discussion of potential error sources, e.g. errors of the analytical data and the calibrations of the models, and takes into account the complex metamorphic history of the rocks and the kinetics of the mineral equilibria.P-T estimates were obtained for seven subareas from the rim compositions of the coexisting minerals: Shevaroy Hills 680±55°C—7·4±1 kb; Kollaimalai area 680±40°C—8·6± 1 kb; Nilgiri Hills 680±90°C—6·6±0.8kb (upland massif) and 705±60°C—9·3±0.8 kb (northern margin); Bhavani Sagar area 650±50°C—7·2± 1 kb; Sargur-Mysore area 690±60°C—7·6 kb; Bangalore-Kunigal-Satnur area 760±50°C—6 kb. Except for the last subarea, the P-T model data reflect the conditions of a late annealing stage probably related to the retrogressive metamorphism. Conditions near the peak of granulite facies metamorphism (730–800°C—6·5–9·5 kb) are recorded by the core compositions of the minerals. Although a rather uniform cooling history of the main part of the charnockite-khondalite terrane is suggested from the temperature data, differential uplift of smaller blocks is indicated by the regional variation of the pressure data.

High-pressure metamorphism in mafic schist from northern Vermont

The only confirmed occurrence of glaucophane and omphacite in the Appalachian Mountains of New England is in mafic schist from Tillotson Peak (44°48', 72°33'), north-central Vermont. Mineral assemblages observed, all with epidote + sphene ± magnetite ± phengitic muscovite ± carbonate (calcite or dolomite) ± apatite ± sulfide, are: (1) actinolite + glaucophane + chlorite + garnet + quartz, (2) actinolite + glaucophane + chlorite + albite + garnet, (3) actinolite + glaucophane + omphacite + garnet, (4) glaucophane + chlorite + garnet+ quartz, (5) glaucophane + omphacite + quartz, (6) albite + glaucophane + chlorite + quartz, and (7) albite + paragonite + chlorite + quartz. Electron microprobe investigations show that plagioclase, chlorite, sphene, white mica, carbonate, and magnetite grains are relatively homogeneous. However, amphibole, pyroxene, epidote, and garnet grains are zoned, indicating incomplete equilibration and changing physical conditions during metamorphism. Glaucophane in assemblage (6) has the general formula (rim composition): □ (Na_(1.9^(M4)Ca_(0.1)) (Mg_(1.8Fe_(1.2)^(2+)Al_(1.0)^(VI)Fe_(0.3)^(3+)) (Si_(7.9)Al_(0.1)^(IV)) O_(22) (OH)_(2•) Glaucophane in assemblages (1) to (5) is somewhat poorer in the glaucophane endmember and richer in actinolite. Ca-rich amphibole grains are zoned extensively with barroisite cores and actinolite rims. Omphacite grains in assemblages (3) and (5) have the general formula (rim composition) (Na_(0.5)Ca_(0.5)) (Mg_(0.4)Fe_(0.1)^(2+)Al_(0.4)^(VI)Fe_(0.1)^(3+)) Si_2O_6. In spite of the significant zoning, the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition. Correlation of mineral compositions and assemblages with mineral stability data suggests high-pressure facies series metamorphism at about 9 ± 2 kb and 450 ± 100°C with P_(h2O) ≈ P_(Total). It is proposed that the metamorphism occurred during the Ordovician.

Metamorphic zones and physical conditions of metamorphism in Leros Island, Greece

On the basis of the systematic variation and the appearance and disappearance of some metamorphic minerals in metapelitic assemblages, the metamorphic terrain of Leros can be divided into chlorite, biotite, garnet and staurolite-kyanite zones of progressive regional metamorphism. The matapelites are interbedded with blueschists containing magnesioriebeckite in Fe3+-rich mafic assemblages in the chlorite zone and “more normal” greenschist and amphibolite facies in higher grade zones. Combining the observed mineral assemblages in pelitic and mafic schists with the available experimental or calculated relevant phase equilibria, one can deduce temperature conditions of metamorphism ranging from about 350° C up to about 700° C and pressures ranging between a minimum value defined by the pressure of the triple point of the Al2SiO5 polymorphs and a possible minimum around 7 kb.

Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane

On Santa Catalina Island, blueschist is structurally overlain by glaucophanic greenschist, which is overlain in turn by a unit of amphibolite and ultramafic rock. These three units are juxtaposed along sub-horizontal postmetamorphic thrusts; tectonic blocks of amphibolite are distributed along the thrust between the greenschist and the blueschist. Physical conditions of metamorphism are estimated to be approximately 300°C and 9 kb for blueschist, 450°C and 8 kb for greenschist, and 600°C and 10 kb for amphibolite. I suggest that metamorphism occurred in a newly started subduction zone, where an inverted thermal gradient developed below the hot hanging-wall peridotite. Postmetamorphic eastward underthrusting along surfaces of varying dip can explain the present structural relationships. Tectonic blocks of glaucophane-epidote schist, amphibolite, and eclogite elsewhere in the Franciscan Complex may be disrupted remnants of similar metamorphic zones. The inverted thermal gradient will only exist in the early stages of subduction, which explains why the blocks are the oldest rocks in the Franciscan Complex. The gross decrease in age and metamorphic grade westward across the Franciscan results from successive underthrusting and accretion of progressively younger slices of supercrustal material, concurrent with uplift and erosion. Pressure-temperature (P-T) conditions of metamorphism in each east-dipping tectonic slice will increase down-dip. At any given time, older, more easterly slices will have been uplifted further, hence metamorphic grade in the exposed edges will increase eastward and structurally upward. If erosion is faster than accretion for a time, younger slices will be metamorphosed at lower pressures than were the older higher ones. Simple reverse faulting can then produce the observed interleaving of rocks of different metamorphic grade.

Geology of a Portion of the North-Central Venezuelan Andes

The stratigraphy, petrology, metamorphism, and structural geology of a portion of the glaciated central Venezuelan Andes were studied in an attempt to decipher the metamorphic and tectonic history of the region. Three distinct rock sequences were recorded: (1) the amphibolite grade metamorphics of the Sierra Nevada Formation, (2) the greenschist grade metasediments of the El Aguila Formation, and (3) the sedimentary rocks of the Cretaceous sequence of western Venezuela. The principal rock types of the Sierra Nevada Formation are: (a) quartzo-feldspathic mica schists, sillimanite-bearing assemblages, and rare staurolite; (b) micaceous quartzo-feldspathic gneisses with rare sillimanite and garnet; and (c) hornblende-epidote amphibolites. All three major rock types (a,b,c) are believed to represent metamorphosed sedimentary rock. The formation is divided into an upper and a lower member on the basis of the presence or absence of amphibolite in the section. The unit is at least 5 km thick. The El Aguila Formation is divided into three members: (d) the Gavilan Quartzite Member, a thinly laminated, fine-grained meta-quartzite; (e) the El Balcon Member, a unit of interbedded phyllites and metasiltstones; and (f) the Cebollata Limestone Member, amassive unit of thinly laminated siliceous limestones. Staurolite and andalusite are locally prominent minerals in the pelitic layers. The total thickness of the formation is about 1.8 km. Formations of the well-known Cretaceous sequence of western Venezuela are seen in the northern portions of the area. Two major granitic bodies intrude the area: (g) the El Carmen Granodiorite, characterized by a quartz-plagioclase-microcline-(mica) assemblage; and (h) the La Culata Adamellite, a leucocratic plagioclase-microcline-quartz-(mica) rock. Minor granitic and pegmatitic bodies are common. Three deformational periods are recorded, as shown by cross-folding and by lineation and S-surface stereoplot analysis. The two major faults in the area are the high-angle reverse Gavilan fault and the high-angle (strike-slip ?) Bocono fault. Comparison of observed mineral assemblages with experimental data on silicate minerals allows an estimation of the boundary values of the physical conditions of metamorphism. For the Sierra Nevada metamorphism, T max is estimated as 620° to 710°C at pressures estimated from 4.3 to 7.0 kbars. For the El Aguila Formation, metamorphism T max may have reached 400°C; T min exceeded 210°C and probably 310°C at pressures which probably did not exceed 1kbar. Local modification of the general El Aguila metamorphism permitted the appearance of staurolite and andalusite. The inferred sequence of events in the area is: (1) the deposition of the Sierra Nevada Formation in the (late ?) Precambrian; (2) D 1 deformation in (probably) the late Precam-brian; (3) intrusion of the La Culata batholith in the early Paleozoic; (4) deposition of the El Aguila Formation in Permo-Carboniferous time; (5) D 2 deformation and the intrusion of the El Carmen Granodiorite (and perhaps the La Culata batholith) about 200 m.y. ago; (6) deposition of the Cretaceous units; and (7) the post-Eocene D 3 deformation, involving largely uplift and tilting.

Widespread occurrence of jadeite, lawsonite, and glaucophane in central California

The Pacheco Pass area, 80 mi SE. of San Francisco, has the most extensive development of metamorphic jadeite and lawsonite yet reported. The host rocks are metamorphosed geosynclinal sedimentary and igneous rocks in the Franciscan Formation (Jura-Cretaceous), and are of 2 types: 1) partially recrystallized rocks that appear in hand specimen relatively unaltered ("subcrystalline" rocks); and 2) thoroughly recrystallized schists, gneisses, and hornfelses ("crystalline" rocks). Subcrystalline rocks predominate; crystalline rocks are common locally, especially in major shear zones. Microscopic and X-ray analyses of the subcrystalline rocks indicate a progressive metamorphic sequence not apparent elsewhere. Lawsonite crystallized first, replacing calcium plagioclase, muscovite, biotite, and possibly other minerals. It occurs in almost all samples from the eastern half of the map area (140 sq mi), and in approximately one-half of those from the western part. Jadeite crystallized next, replacing albite in clastic sedimentary rocks, and both albite and augite in greenstones. Glaucophane, replacing chlorite and hornblende, began to crystallize typically before the crystallization of jadeite was completed. Jadeite and glaucophane are present in almost all samples from the eastern half of the area. Jadeite, lawsonite, and glaucophane are common also in the crystalline rocks, along with other minerals characteristic of the glaucophane schist facies. Greenschist facies rocks and eclogite occur locally in shear zones. The consistent distributional pattern of the subcrystalline rock types probably reflects a regional variation in the physical conditions of metamorphism. The local development of crystalline rocks might indicate sharp local changes in temperature, pressure, or solution action, or possibly the carrying in of these rocks by faulting. The high P-T condition of metamorphism may have been caused by rapid subsidence in a deep marine environment, coupled with widespread tectonic pressure. The absence of syntectonic intrusions probably reflects the low temperature condition of metamorphism, but whether this is a cause or effect relationship is not clear. Finally, the widespread nature of this type of metamorphism at Pacheco Pass suggests new possibilities for seismic interpretation of parts of the crust which may have had a similar geologic history.