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1.
M. ENAMI 《Journal of Metamorphic Geology》1983,1(2):141-161
The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa = 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH2 O = Psolid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as: 相似文献
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as: 相似文献
2.
S. BANNO 《Journal of Metamorphic Geology》1998,16(1):117-128
The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes 相似文献
3.
The prograde amphibole that coexists with chlorite, epidote, muscovite, albite, quartz and hematite in Sanbagawa schists was examined to investigate the relationship between the prograde P-T paths of individual rocks and the metamorphic field gradient in the Sanbagawa metamorphic belt, central Shikoku. The amphibole changes from actinolite, through ferri-winchite and crossite, to barroisite and hornblende with increasing grade along the metamorphic field gradient. However, the sequence of prograde amphibole compositions in each sample varies in different mineral zones. The general scheme can be summarized as: magnesioriebeckite-riebeckite crossite in the upper chlorite zone of lower-grade rocks; crossite or glaucophane barroisite in the garnet zone of medium-grade rocks; and actinolite or winchite barroisite hornblende in the albite-biotite zone of higher-grade rocks. Changes of amphibole composition indicate that the prograde P-T path recorded in the higher-grade rocks was situated on the higher-temperature side of that of the lower-grade rocks and on the lower-pressure side of the metamorphic field gradient. The systematic change of P-T paths implies an increasing d P /d T during continuous subduction. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non-steady-state geotherm. 相似文献
4.
PAUL A. STUDEMEISTER 《Journal of Metamorphic Geology》1985,3(1):79-90
Abstract Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite is partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite + white mica + calcite + chlorite near fracture zones traversing the stocks.
The alteration of the Gutcher Lake stock along its foliated margin involved addition of K2 O, H2 O + CO2 , MnO, plus Rb; loss of CaO plus Sr; and a shift in Fe+2 /Fet from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H2 O + CO2 ; loss of Sr; and no significant shift in Fe+2 /Fet . The greenschist alteration also modified the contact aureoles bordering both stocks.
One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron. 相似文献
The alteration of the Gutcher Lake stock along its foliated margin involved addition of K
One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron. 相似文献
5.
E. H. Brown 《Contributions to Mineralogy and Petrology》1977,64(2):123-136
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
- lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
- lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
- crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
- crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
- calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
- pumpellyite + chlorite + quartz = epidote + actinolite + H2O
6.
In central Shikoku, SW Japan, the Mikabu belt is bounded to the north by the Sanbagawa belt, and to the south by the northern (N) Chichibu belt. The N-Chichibu belt can be further subdivided into northern and southern parts. There is no apparent difference in the overall geology, structure, or fossil and radiometric ages between the Mikabu belt and the northern part of the N-Chichibu belt. Greenstones from the Mikabu belt and the northern part of the N-Chichibu belt show evidence for similar low-grade metamorphism, and include the following mineral assemblages with albite+chlorite in excess: metamorphic aragonite, sodic pyroxene+quartz, epidote+actinolite+pumpellyite, glaucophane+ pumpellyite+quartz, and lawsonite (not with actinolite or glaucophane). These similarities suggest that the Mikabu belt and the northern part of the N-Chichibu belt belong to the same geological unit (the MB-NNC complex). The mineral assemblages also indicate that the MB-NNC complex belongs to a different metamorphic facies from the low-grade part of the Sanbagawa belt, that is, the former represents lower temperature/higher pressure conditions than the latter. Structural and petrological continuity between the MB-NNC complex and Sanbagawa belt has not yet been confirmed, but both have similar radiometric ages. It is therefore most likely that the MB-NNC complex and Sanbagawa belt belong to the same subduction complex, and were metamorphosed under similar but distinct conditions. These two units were juxtaposed during exhumation. In contrast, the southern part of the N-Chichibu belt is distinct in lithology and structure, and includes no mineral assemblages diagnostic of the MB-NNC complex and the Sanbagawa belt. Thus, the southern part of the N-Chichibu belt may represent a different geological unit from the MB-NNC complex and Sanbagawa belt. 相似文献
7.
Yasuyuki Banno 《Lithos》2000,50(4):289-303
The retrograde chemical zonal structure of amphibole in hematite-bearing basic and quartz schists from the higher grade zone in the Saruta-gawa area of the Sanbagawa belt was studied to investigate the relationships between the prograde and retrograde P–T paths of the Sanbagawa metamorphism. This amphibole coexists with chlorite, epidote, muscovite, albite, quartz and hematite, and is composed of Al-rich core and Al-poor mantle. The core is fairly homogeneous and has a barroisitic composition. In the mantle part, [B]Na increases with decreasing [4]Al towards the margins, which have winchite–magnesioriebeckite compositions. The barroisite–winchite–magnesioriebeckite composite crystal is sometimes rimmed by actinolite and/or winchite with low [4]Al and [B]Na. The Al-rich core and Al-poor mantle are regarded as prograde and retrograde products, respectively. The retrograde mantle in the Saruta-gawa area: (1) is systematically richer in [B]Na [0.40–1.73 per formula unit (pfu; for O=23)] than that from the same grade zone in the Asemi-gawa area (0.19–0.78 pfu), about 8 km south of the studied area; (2) tends to be [B]Na-poorer (less than 1.73 pfu) than prograde sodic amphibole (up to 1.93 [B]Na pfu) produced in the peak temperature stage from the lower grade zone in the same and other areas; and (3) extends its compositional range towards higher [B]Na and lower [4]Al than prograde-formed amphibole from the same grade zone in the same area. These zonal characteristics imply that (1) the Saruta-gawa samples experienced retrograde metamorphism under higher P/T conditions than the Asemi-gawa samples, (2) the retrograde P–T path of the Saruta-gawa area passes on the lower pressure side of the metamorphic field gradient, and (3) the Saruta-gawa samples underwent retrograde metamorphism under higher P/T conditions than the prograde metamorphism. The higher P/T conditions of the retrograde metamorphism suggests an increasing dP/dT of the geotherm during exhumation. Retrograde P–T conditions during the formation of magnesioriebeckite can be roughly estimated at 7–8 kbar, 400–450°C based on semi-quantitative phase relations of actinolite–winchite–magnesioriebeckite–barroisite series associated with chlorite, epidote, muscovite, albite, quartz and hematite. 相似文献
8.
A new petrogenetic grid for low-grade metabasites 总被引：7，自引：0，他引：7
Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na2 O-CaO-MgO-Al2 O3 -SiO2 -H2 O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition. 相似文献
9.
Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku 总被引：2，自引：0，他引：2
The mineral assemblages of hematite-bearing basic schists in intermediate high-pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four-component system, without omitting or grouping major components.
In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite-bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low- T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P-T trajectory which passes on the lower pressure side of the prograde one. 相似文献
In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite-bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low- T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P-T trajectory which passes on the lower pressure side of the prograde one. 相似文献
10.
北祁连山硬柱石蓝片岩p-T条件相平衡计算及其岩石学意义 总被引：2，自引：0，他引：2
北祁连硬柱石蓝片岩主要分布在甘肃省肃南县九个泉一带,是目前中国唯一报道的、确切地含有硬柱石的蓝片岩。文中在详细的岩石学和矿物学研究基础上,根据矿物共生组合的不同,将北祁连低温蓝片岩进一步划分为绿纤石蓝片岩、硬柱石蓝片岩和绿帘石蓝片岩。绿纤石蓝片岩的特征变质矿物组合为蓝闪石(>40%)+绿纤石(30%)+绿泥石(10%)+钠长石(8%)+石英(5%)+硬柱石(<3%)±方解石/文石(<1%)。硬柱石蓝片岩的矿物组合为蓝闪石(35%~40%)+硬柱石(35%~40%)+绿泥石(10%)+钠长石(10%)+石榴石(1%~2%)+黝帘石/斜黝帘石(<2%)+石英(<1%),副矿物有磷灰石和榍石,总含量小于2%。绿帘石蓝片岩的矿物组合为蓝闪石(30%~35%)+黝帘石/斜黝帘石/绿帘石(~30%)+绿泥石(15%)+钠长石(15%)+石榴石(2%)+石英(<2%),副矿物有金红石、磷灰石和磁铁矿,总含量小于2%。利用矿物内部一致性热力学数据和Domino/Theriak软件计算了这三种类型的蓝片岩形成的峰期温压条件,它们分别是绿纤石蓝片岩为320~350℃,0.75~0.85GPa;硬柱石蓝片岩为335~355℃,0.8~0.95GPa;绿帘石蓝片岩为345~375℃;0.75~0.85GPa。北祁连低温蓝片岩带由硬柱石蓝片岩相到绿帘石蓝片岩相的转化代表了俯冲变质过程中的递进变质过程。 相似文献
11.
Kazuaki Okamoto Hironao Shinjoe Ikuo Katayama Kentaro Terada Yuji Sano Simon Johnson 《地学学报》2004,16(2):81-89
In order to decipher the origin of eclogite in the high‐P/T Sanbagawa metamorphic belt, SHRIMP U–Pb ages of zircons from quartz‐bearing eclogite and associated quartz‐rich rock (metasandstone) were determined. One zircon core of the quartz‐rich rock yields an extremely old provenance age of 1899 ± 79 Ma, suggesting that the core is of detrital origin. Eight other core ages are in the 148–134 Ma range, and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139–135 Ma from the Sanbagawa schists. Ages of metamorphic zircon rims (132–112 Ma) from the quartz‐rich rock are consistent with metamorphic zircon ages from the quartz‐bearing eclogite, indicating that eclogite facies metamorphism peaked at 120–110 Ma. These new data are consistent with both the Iratsu eclogite body and surrounding highest‐grade Sanbagawa schists undergoing coeval subduction‐zone metamorphism, and subsequent re‐equilibration under epidote amphibolite facies conditions during exhumation. 相似文献
12.
Decompression-induced growth of albite porphyroblasts, Fleur de Lys Supergroup, western Newfoundland 总被引：2，自引：0，他引：2
Abstract Albite porphyroblasts are widely distributed in pelitic and semi-pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P-T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite-bearing schists, here termed PMAQ (paragonite-muscovite-albite-quartz), MMAQ (microcline-muscovite-albite-quartz), and PMMQ (paragonite-muscovite-margarite-quartz), can be distinguished on the basis of pre-porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a (Na+ /H+ ) versus log a (K+ /H+ )] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre-existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples. 相似文献
13.
Metamorphism and deformation of blueschist belts and their tectonic implications, North Qilian Mountains, China 总被引：25，自引：1，他引：25
Abstract Two blueschist belts in the North Qilian Mountains occur in Middle Cambrian and Lower Ordovician strata and strike N30–35°W for about 500 km along the Caledonian fold belt on the south-west margin of the Sino-Korean plate. The styles of metamorphism and deformation are quite different in the two belts. The Middle Cambrian to Ordovician rocks in the high-grade belt are mainly blueschists and C-type eclogites in which six phases of lower and upper crustal deformation have been recognized. The rocks contain glaucophane, phengite, epidote, clinozoisite, chlorite, garnet, stilpnomelane, piedmontite, albite, titanite and quartz. The estimated P–T conditions of eclogites are 340 ± 10°C, 8 ± 1 kbar and, of blueschist, >380°C, 6–7 kbar. The Ordovician rocks in the low-grade belt are characterized by ductile to brittle deformation in the middle to upper crust. The low-grade blueschists contain glaucophane, lawsonite, pumpellyite, aragonite, albite and chlorite. The estimated P–T conditions are 150–250°C and 4–7 kbar.
K–Ar and39 Ar/40 Ar geochronology on glaucophane and phengite from the high-grade blueschist belt suggest two stages of metamorphism at 460–440 and 400–380 Ma, which may represent the times of subduction and orogeny. The subduction metamorphism of the northern low-grade blueschist belt took place approximately at the end of the Ordovician. 相似文献
K–Ar and
14.
The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd50). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H2Oalbite+chlorite+quartz glaucophane+epidote+H2Oalbite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones.The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate. 相似文献
15.
C. E. JACOBSON 《Journal of Metamorphic Geology》1995,13(1):79-92
Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent-Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic-calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI + Fe3+ + Ti). These variations imply that structurally high rocks belong to a lower-pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.
Amphibole composition is dependent not only on structural position (i.e. P-T ), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe2+ + Mn) and Fe3+ /Fe2+ . The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.
Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial. 相似文献
Amphibole composition is dependent not only on structural position (i.e. P-T ), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe
Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial. 相似文献
16.
Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China 总被引：25，自引：0，他引：25
The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions. 相似文献
17.
A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X CO2 and pressure-temperature (constant X CO2 ) diagrams were calculated to quantify the effects of CO2 in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X co2 <0.002. For X co2 >0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2 O-rich fluid and a more CO2 -rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites. 相似文献
18.
Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na2O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(XFe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO2 values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al2O3content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al2O3 of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al2O3 greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures. 相似文献
19.
The Sanbagawa metamorphic rocks in the Besshi district, central Shikoku, are grouped into eclogite and noneclogite units. Chloritoid and barroisite-bearing pelitic schists occur as interlayers within basic schist in an eclogite unit of the Seba area in the Sanbagawa metamorphic belt, central Shikoku, Japan. Major matrix phases of the schists are garnet, chlorite, barroisite, paragonite, phengite, and quartz. Eclogite facies phases including chloritoid and talc are preserved only as inclusions in garnet. P–T conditions for the eclogite facies stage estimated using equilibria among chloritoid, barroisite, chlorite, interlayered chlorite–talc, paragonite, and garnet are 1.8 GPa/520–550 °C. Zonal structures of garnet and matrix amphibole show discontinuous growth of minerals between their core and mantle parts, implying the following metamorphic stages: prograde eclogite facies stage→hydration reaction stage→prograde epidote–amphibolite stage. This metamorphic history suggests that the Seba eclogite lithologies were (1) juxtaposed with subducting noneclogite lithologies during exhumation and then (2) progressively recrystallized under the epidote–amphibolite facies together with the surrounding noneclogite lithologies.
The pelitic schists in the Seba eclogite unit contain paragonite of two generations: prograde phase of the eclogite facies included in garnet and matrix phase produced by local reequilibration of sodic pyroxene-bearing eclogite facies assemblages during exhumation. Paragonite is absent in the common Sanbagawa basic and pelitic schists, and is, however, reported from restricted schists from several localities near the proposed eclogite unit in the Besshi district. These paragonite-bearing schists could be lower-pressure equivalents of the former eclogite facies rocks and are also members of the eclogite unit. This idea implies that the eclogite unit is more widely distributed in the Besshi district than previously thought. 相似文献
20.
EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan.
Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation.
K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains. 相似文献