Epidote Blueschist Research Articles

Blueschist-to-Greenschist Transition and the P-T Path of Prasinites from the Lavrion Area, Greece

AbstractThe Lavrion area is part of the Attic-Cycladic massif. Blue amphibole analyses revealed that they are glaucophane or ferroglaucophane. Ca-amphiboles are characterized as actinolite or actinolitic hornblende. The Ps component of epidotes from the glaucophane-bearing rocks varies from 25.42–30.89%, whereas the Ps component of epidotes from the greenschist assemblages ranges from 23.81–26.88%. Chlorites show narrow compositional variations of ferromagnesian ratios (Xmg=0.48–0.53). Albites are almost pure Ab100. The Kfeldspar present has a low Ab content. The evolution of the prasinites studied is characterized by the progressive transformation of eclogite facies (?) rocks through epidote blueschists into greenschists. A P-T path for the prasinites is presented indicating epidote blueschist facies at P-T conditions of around 7.0–7.5 kbar and 300–340°C. Pressures of 4.0–4.5 kbar and temperatures around 340–360°C are estimated for the subsequent overprint in the greenschist facies. The path EBS to GS conditions followed under nearly isothermal uplift.

Petrology, Metamorphic Process and Genesis of The Dabie—Sulu Eclogite Belt, Eastern—Central China

Abstract The Qinling—Dabie—Sulu high—pressure and ultra—high pressure metamorphic belt was formed by subduction and collision between the North China and Yangtze plates. The study of the eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. In the present paper the geology, petrology, minerology and chronology of the eclogites in the Dabie and Sulu areas are described. The principal conclusions of this work are as follows: (1) Based upon the field occurrence and the P—T conditions of the eclogites, two types of eclogite can be distinguished: Type 1—the low‐temperature and high—pressure eclogite in the mid‐late Proterozoic metamorphic series, and Type 2—the ultra‐high pressure eclogite in the late Archaean to early Proterozoic metamorphic complex. In the Dabie area, the ultra‐high‐pressure eclogite, high—pressure eclogite and epidote‐blueschist units are nearly parallel to each other and stretch intermittently from north to south. (2) The P—T conditions of the high—pressure eclogites and ultra‐high pressure eclogites have been estimated. The former are formed at 450—550°C and 1.4‐1.6 GPa; while the latter at 650—870°C and >2.7—2.9 GPa in the Dabie area and at 820—1000°C and > 2.8—3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogites increase progressively from west to east. (3) The ultra—high pressure eclogites were subjected to 5 stages of metamorphism: pre‐eclogite epidote amphibolite facies, peak coesite eclogite facies, post—eclogite amphibolite facies, epidote—blueschist facies or epidote amphibolite facies and greenschist facies. The general features of the PTt path of the ultra‐high pressure eclogite are: clockwise pattern, progressive metamorphism being a process of slow increasing temperature and rapid increasing pressure, and the retrogressive section with nearly isothermal decompression at the early stage, isobaric cooling at the middle stage and nearly isothermal decompression at the late stage. (4) At least two stages of high‐pressure metamorphism occurred in the orogenic belt: the high‐pressure eclogite and ultra‐high pressure eclogite were formed by the subduction of the oceanic crust northward beneath the North China plate or the Dabie block during the Caledonian; while the epidote‐blueschist belt came into being by subdution and collision between the two continental plates during the Indosinian. (5) Due to the continuous sequential subduction of the cold plate, the ultra high—presssure metamorphic rocks were uplifted to the crust by the underplating processes. They can be preserved just because of the “frozen effect” resulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite, breunnerite, aragonite and dolomite, and the H2O—bearing minerals, such as phengite, epidote and zoisite, were stable during the high‐pressure and / or ultra—high pressure metamorphism.

40Ar/39Ar thermochronology of epidote blueschists from the North D'Aguilar block, Queensland, Australia: Timing and kinematics of subduction complex unroofing

Mechanisms proposed for exhumation of blueschists include whole-crust uplift and erosion, denudation by crustal-scale extensional faulting or ductile strain, and transport of blocks entrained in rising diapiric masses. Insight into the unroofing process, and the role of granitic plutonism, can be gleaned from study of North D9Aguilar block in the New England orogen of southeastern Queensland, Australia. Epidote blueschist facies rocks occur as structurally coherent schists and as blocks in serpentinite matrix melange. These subduction-underplated rocks formed at depths >18 km and occur in the lower plate of a metamorphic core complex. A slate from the upper plate has a whole-rock total-gas age of 315 Ma, interpreted as a minimum age for subduction along this part of the paleo-Pacific margin. Exhumation of lower plate schists was coeval with overprinting of early high-pressure (M 1 ) metamorphic fabrics by a greenschist facies fabric (M 2 ) and was accomplished in part by ductile stretching and normal faulting. Phengites from most lower plate schists yield 40 Ar/ 39 Ar plateau ages of ca. 299–296 Ma, recording the time elapsed since the schists cooled below ∼350 ± 50 °C following M 2 . Plateau-shaped age spectra and small structural depth-related age gradients indicate rapid cooling of schists in the footwall of a present-day low-angle normal fault. Samples from deeper levels of the lower plate remained at >350 °C for 35 m.y. longer than the other samples and contain extraneous Ar possibly absorbed from fluids originating at depth. At ca. 307 Ma a granodiorite pluton was intruded into the lower plate near the peak of M 2 . A synkinematically intruded, in part mylonitic pluton increased the regional M 2 fabric to amphibolite facies in its aureole. Heat from the granitic pluton, and perhaps others like it at depth, may have played a strain-softening role in formation of the core complex. Similar 40 Ar/ 39 Ar cooling ages for different blueschist and greenschist blocks in serpentinite matrix melange support the view that the Australian melange was uplifted by extensional tectonic processes unrelated to serpentinite diapirism.

Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist

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.

Polyphase and anticlockwise P‐T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

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) at c. 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 anticlockwise P‐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 anticlockwise P‐T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre‐Franciscan metamorphism.

Infiltration vs. thermal overprinting of epidote blueschists, Ile de Groix, France

Epidote blueschists on Ile de Groix, France, were selectively over-printed by chloritic greenschist assemblages on a millimetre to metre scale along foliation planes, in fold hinges, and along shear zones. Reactions balanced between the blueschist and greenschist assemblages show that overprinting of epidote blueschists can only be accomplished by the addition of H 2 O ± CO 2 Conversion of epidote blueschists to chloritic greenschist assemblages thus depends on fluid-infiltration processes as the rocks pass through the greenschist facies, and does not necessarily reflect rapid rates of unroofing of a subduction complex. Epidote blueschists are therefore preferentially preserved in fluid-absent or impermeable rocks regardless of exhumation rates, in contrast to other blueschist types that require rapid unroofing to avoid a greenschist overprint.

Cooling during the exhumation of a blueschist terrane: Sifnos (Cyclades), Greece

Alpine high-pressure metamorphic rocks exposed in the Cyclades were overpinted during their exhumation. We studied the metamorphic evolution of a pervasively overprinted rock sequence, the greenschist unit, exposed in the island of Sifnos. We show that the metamorphic evolution in the Greenschist unit is characterized by the progressive transformation of eclogite-facies rocks through albite-bearing epidote blueschists into greenschists. The evolution of the mineral assemblages during exhumation is shown by a series of chemographic diagrams which are referenced to specific P-T fields in the petrogenetic grid proposed by Evans (1990). P-T conditions corresponding to the major paragenetic groups are estimated also on the basis of mineral chemistry, Fe/Mg exchange thermometry and the use of GEOCALC-PTX and THERMOCALC softwares. A P-T path for the Greenschist unit is presented indicating eclogite-facies conditions of around 480-520 ° C and 12-15 kbar, followed by overprinting in the albite-bearing epidote-blueschist facies at P-T conditions of 8-10 kbar and ca. 480-500 ° C. Pressures of 5.5 kbar and temperatures around 450 ° C are estmated for the subsequent overprint in the greenschist facies. The decompression part of the P-T path is characterized by cooling with no indication of a temperature rise. Thus, the high-pressure metamorphic rocks of Sifnos appears not to follow the thermal evolution path predicted by models which assume whole crust uplift and erosion. On a regional scale the exhumation of the Cycladic blueschist belt is characterized by cooling in the west, but broad clockwise P-T loops with significant heating during decompression are found in the east. We note that cooling affected rocks situated at the deepest levels of the Alpine orogenic wedge (Sifnos, Syros, Tinos) whereas heating has affected rocks exhumed from shallower structural levels (Naxos, Ios)

The Sistan suture zone of eastern Iran

A deformed accretionary prism and a flanking forearc basin extending from Birjand southeast to Zahedan, Iran, record the destruction of an arm of the Neo-Tethys during Senonian-Paleocene time and consequent collision of the Afghan and Lut eratonic blocks. The accretionary prism at 32 °N is subdivided into two northwest-trending en echelon belts termed the “Ratuk” and “Neh” complexes, respectively. On the east, the Ratuk complex is characterized by ophiolitic block-against-block or serpentinite-matrix melange and large fault slivers of epidote blueschist tectonite. The Ratuk complex was built prior to Maastrichtian time. The Neh complex to the southwest is Senonian to Eocene in age and includes, in addition to ophiolitic melange, weakly metamorphosed marine sedimentary rock exposed in extensive belts bounded by steeply dipping faults. The Sefidabeh forearc basin deposits onlap both the Neh and Ratuk complexes and the southwest margin of the Afghan block. They make up as much as 8 km of Cenomanian to Eocene terrigenous elastics and carbonates that display a complex but coherent stratigraphy. Facies relations demonstrate the uplift and subaerial exposure of the Ratuk structural high, followed by its subsidence contemporaneous with construction of the Neh complex and calc-alkalic volcanism on the northeast (inner) side of the basin. The accretionary prism-forearc basin polarity, the structural vergence and general younging of the accretionary prism to the southwest, as well as the position of the (relatively) high P T metamorphic rock on the inner side of the prism are consistent with northeast-dipping subduction. Widespread emergence of the entire belt and the initiation of folding of the Sefidabeh basin deposits during middle Eocene are interpreted to be consequences of the entry of the Lut block into the subduction zone. Continued convergence of the continental blocks is expressed by a regional system of folds and transcurrent faults corresponding to east-northeast compression. These structures are buried by mildly deformed Miocene volcanic rocks. Extensive post-Miocene right-slip faulting is inferred to be an effect of Miocene “terminal” collision of Arabia and Eurasia.