摘要:

ABSTRACTHigh‐grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well‐defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) atc. 90d̀ to S1, with syntectonic growth of garnet, defining the (albite‐)epidote‐amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite‐)epidote‐amphibolite/(garnet‐bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well‐defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post‐D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry.The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwiseP‐Tpath: (1) epidote blueschist (garnet‐free) to (2) (albite‐)epidote‐amphibolite to (3) transitional epidote blueschist (garnet‐bearing)/(albite‐)epidote‐amphibolite to (4) eclogite to (5) epidote blueschist (garnet‐bearing) to (6‐7) epidote blueschist (garnet‐free) facies to (8) lawsonite + pumpellyite + aragonite‐bearing assemblages. This anticlockwiseP‐Tpath may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of Califor

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00008.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTPorphyroblast inclusion trails have the potential to provide critical information about tectonometamorphic events. Recently, however, traditional interpretations of inclusion trails have been called into question by the suggestions that porphyroblasts do not rotate during non‐coaxial deformation and that apparent spiral inclusion trails can be generated in coaxial deformation. We present a new computer model that simulates inclusion trail development. Model results suggest: (1) that the extent of porphyroblast rotation is controlled by conditions at the porphyroblast‐matrix boundary; (2) that curved inclusion trails may develop in unrotated porphyroblasts; (3) that classic “snowball” inclusion trails are most simply explained by rotational growth histories; and (4) that some of the observations used to support the view that porphyroblasts do not rotate (e.g. weakly sigmoidal inclusion trails, apparent truncations of inclusion trails) can be accounted for by variations in the growth rate of rotating porphyr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00009.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTClay mineral crystallinity and crystallite (domain) size data determined by X‐ray diffraction (XRD) are methods extensively used in the characterization of very low‐grade metamorphic conditions. However, the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained, a consequence of the different machine conditions, measurement methods and sample preparations used during analysis. A calibration approach to the standardization of data using rock chip standards is presented, which allows data sets produced by different research groups to be directly and quantitatively compared. A standardized scale, the crystallinity index standard (CIS), is proposed, with illite crystallinity anchizonal boundary limits of 0.25d̀Δ2θ and 0.42d̀Δ2θ, and equivalent illite crystallite sizes of 52 and 23 nm, respectively, determined by the Warren‐Averbach method. Calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments, and should improve the accuracy and reliability of correlations made between XRD data and other indicators of very low‐grade metamorphism, hence increasing the value of such clay mi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00010.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2‐3 kbar. The bulk rock compositions of metapelites are aluminous withA′values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low‐grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al2SiO5polymorphs is limited in typical pelitic rocks.The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K‐feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite‐sillimanite type aureoles. Rocks with FeO/(FeO + MgO)>0.8 progressively develop chloritoid‐bearing assemblages from Bt‐Chl‐Cld, And‐Bt‐Cld, to And‐Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low‐temperature side of the formation of cordierite by the terminal equilibrium of chlorite. AP‐Tmodel depicting the relative stability of chloritoid and staurolite at low‐ and medium‐pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, ch

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00011.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTUltramafic blocks that themselves contain eclogite lenses in the Triassic Su‐Lu ultrahigh‐Pterrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al2O3(<3.8wt%), CaO (<3.2%) and TiO2(28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh‐pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision betwee

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00012.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTRocks within the Zermatt‐Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm‐Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd‐rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm&Nd is thought to be>600d̀C, resetting due to post‐high‐pressure diffusion is thought to be insignificant. Given the fine‐grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre‐metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high‐pressure metamorphism in the Cretaceous n

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00013.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA breccia vein sampled from a shear zone in greenschist facies metapelites at Mount Isa, Queensland, Australia, shows a systematic variation in vein geometry that is related to the geometry of folding and faulting within the sample. Calcite vein‐fill is coarse grained and equigranular, suggesting precipitation in a fluid‐filled space. Partially folded veins suggest that veining occurred during folding and faulting. The breccia vein contains a central zone in which dilation has occurred simultaneously in all directions in the plane of section, implying that this was a zone of high fluid pressure and nearly isostatic differential stress during folding and faulting. From these observations, it can be inferred that the breccia vein was a zone of high permeability and a likely fluid channel during deformation. This hypothesis was tested by stable isotope analysis of veins and host rocks. The calcite veins have δ13C values of ‐11.1 ± 0.1% and δ18O values of 6‐10%o, whereas the host metapelite has δ13C values of ‐10.62 and ‐10.11% and δ18O values of 14‐15%o. These values are consistent with an igneous‐derived, H2O‐dominated fluid that exchanged little oxygen with the host rocks, but derived much of its carbon from the wall rock. The isotopic disequilibrium between the veins and the wall rock confirms that the fluid was externally derived, and that the breccia vein acted as a channel for large‐volume fluid fl

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00014.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00015.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY