The antigorite–grade serpentinite and Late Paleozoic high–pressure schists are main components of a serpentinite–matrix mélange in the Itoigawa–Omi area, Hida–Gaien Belt, Japan. Based on the ...petrologic characteristics of the high–pressure schists, the mélange is divided into two units, namely an ‘eclogitic unit’ and a ‘non–eclogitic unit’. Our preliminary in–situ boron isotope analyses of five serpentinites using a laser–ablation multiple–collector inductively–coupled–plasma mass spectrometry (LA–MC–ICPMS) found a systematic difference of boron isotopic trends among the two units in the same mélange. The ‘eclogitic unit’ serpentinites from Yunotani and Kotagi–gawa are characterized by lower δ11B value (mostly lower than +10‰), whereas the non–eclogitic unit serpentinite from Omi–gawa is higher than +10‰. Although the δ11B value of <0‰ was not measured from the eclogitic unit serpentinites, the relatively low δ11B values of <+10‰ might have recorded the signature of fluids released from deep subducted dehydrating slab. In contrast, the non–eclogitic unit serpentinite might have been affected by fluids released from shallower portion. Our new data confirmed the potential sensitivity of the boron isotope signature of serpentinites reflecting variation of high–pressure metamorphism.
The following is errata for the original article entitled “Revisiting Pb isotope signatures of Ni-Fe alloy hosted by antigorite serpentinite from the Josephine Ophiolite, USA” by KAKEFUDA, Mayu, ...TSUJIMORI, Tatsuki, YAMASHITA, Katsuyuki, IIZUKA, Yoshiyuki and FLORES, Kennet E. (Vol. 115, no. 1, 21–28, 2020).The authors apologize for these errors and any confusion these may have caused. We would like to thank Dr. Akira Kato for kindly pointing out the mistakes. Page 24, Figure 2(c): A label of ‘NiAs’ should be replaced with ‘Ni-As’.Page 24, Figure 2 caption: The explanations of mineral abbreviations and a note should be added.
The Guatemala suture zone is a major east–west left-lateral strike slip boundary that separates the North American and Caribbean plates in Guatemala. The Motagua fault, the central active strand of ...the suture zone, underwent two major collisional events within a system otherwise dominated by strike–slip motion. The first event is recorded by high-pressure/low temperature (HP/LT) eclogites and related rocks that occur within serpentinites both north and south of the Motagua fault. Lawsonite eclogites south of the fault are not significantly retrograded and give
40Ar/
39Ar ages of 125–116 Ma and Sm–Nd mineral isochrons of 144–132 Ma. Eclogites north of the fault give similar Sm–Nd isochron ages (131–126 Ma) but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and, along with associated HP/LT rocks, give much younger
40Ar/
39Ar ages of 88–55 Ma indicating a later amphibolite facies metamorphic event. We propose therefore that all serpentinite hosted eclogites along the Motagua fault formed at essentially the same time in different parts of a laterally extensive Lower Cretaceous forearc subduction system, but subsequently underwent different histories. The southern assemblages were thrust southwards (present coordinates) immediately after HP metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca. 70 Ma. They were subsequently juxtaposed opposite each other by major strike slip motion. This model implies that the HP rocks on opposing sides of the Motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the Late Cretaceous as a result of oblique convergence. The juxtaposition of a convergent and strike slip system means that HP/LT rocks within serpentinites can be found at depth along much of the modern Guatemala suture zone and its eastward extension into the northern Caribbean. Both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend. Recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault.
The Mineoka Ophiolite Mélange is located at the intersection of the Pacific, Philippine Sea, Eurasian, and North American plates. The Mineoka ophiolite origin is disputed, and it has been ascribed to ...a fully subducted plate or part of the Pacific and Philippine Sea plates. In this paper, we present a kinematic reconstruction of the Mineoka Ophiolite Mélange and its relation with the Pacific Plate, based on new paleomagnetic data and bulk‐rock 40Ar/39Ar ages of basaltic rocks. In addition to standard analyses for paleolatitudes, we performed a Net tectonic rotation analysis on sheeted dolerite dikes to infer the paleospreading direction that formed the ophiolite. The analysis shows that 85–80 Ma MOR pillow basalts erupted at a paleolatitude of N ∼16°, whereas ∼50 Ma basalts formed at N ∼34°. Net Tectonic Rotation analysis suggests that the spreading direction was NE 60°. Ar–Ar ages yielded 53–49 Ma for MORBs and 41–35 Ma for island‐arc basalts. The formation of this ophiolite occurred in the back‐arc spreading of the Nemuro–Olyutorsky arcs of the NW Pacific. It infers that the final consumption of Izanagi below Japan instigated a subduction jump and flipped its polarity. Subduction initiated parallel to the ridge, and a piece of the original back‐arc crust got trapped near the Japan trench during the northwards motion of the Philippine Sea Plate. The contrasting motion between the Pacific and the Philippine Sea plates generated a highly unstable setting followed by a subduction zone that left a small‐sized and short‐lived plate (“Mineoka”), surrounded by subduction zones.
Key Points
At 85–80 Ma, the Mineoka ophiolite formed in the back‐arc spreading of Nemuro‐Olyutorsky arc, at paleolatitude of N ∼16° in the NW Pacific
The spreading was continued until 49 Ma and placed at paleolatitude of N ∼34°, following the Pacific Plate motion with a NE 60° orientation
A small‐sized, short‐lived “Mineoka” plate existed shortly between the Philippine Sea Plate and the Pacific Plate, subducting below Japan
► The Himalayan HP and UHP eclogites contain two generation (igneous and metamorphic) of zircons. ► Magmatic zircons are oscillatory zoned, have higher U and Th contents (Th/U ratio>1) formed in ...Permian (267Ma). ► Metamorphic zircons are sector zoned, have lower U and Th contents (Th/U ratio<0.04) formed in Eocene (45Ma).
We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100μm up to 500μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6±2.4Ma (MSWD=8.5), have higher U and Th contents with a Th/U ratio>1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9±1.2Ma (MSWD=4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.
Lawsonite, jadeite, and glaucophane are iconic minerals within a Pacheco Pass metagraywacke of the Franciscan Complex, California. Those minerals and the associated quartz form the distinctive very ...low–temperature and high–pressure metamorphic lawsonite–jadeite–glaucophane assemblage, which is diagnostic of ‘cold’ oceanic subduction zones. In this paper, we evaluate the ability of lawsonite geochemistry to trace protoliths with in–situ trace element and Sr–Pb isotope analyses in lawsonite from the Pacheco Pass blueschist–facies metagraywacke, a classical example of trench–fill sediments in subduction zones. Initial Sr isotope ratios are relatively high (87Sr/86Sr = 0.7071–0.7074), and initial Pb isotope ratios are 206Pb/204Pb = 18.74–19.66, 207Pb/204Pb = 15.58–15.70, and 208Pb/204Pb = 38.41–39.34, which range from a MORB trend to a cluster on the EMII component. These geochemical signatures suggest the protolith of the metagraywacke mainly contained material derived from continental volcaniclastic rocks and quartzofeldspathic sediments. There is also a possibility that the protolith contains plume–related oceanic island basalt that reached or intruded into the fore–arc sedimentary sequence of California. Considering the maximum depositional age of the metagraywacke at ~ 102 Ma, the subduction of the Farallon Plate beneath the continental crust of the North American Plate might have carried alkali basalt with OIB–like isotopic signatures to the Franciscan trench. Our study proves the advantage of in–situ lawsonite Sr–Pb isotope analyses to characterize protoliths of metamorphic rocks. The results would manifest that the Sr–Pb isotopic signature of Ca–Al silicate minerals, such as lawsonite, and possibly epidote and pumpellyite, in various types of metamorphic/metasomatic rocks, would be an effective tool for investigating convergent margins.
Greetings from the new Editors‐in‐Chief Kano, Akihiro; Tsujimori, Tatsuki
The island arc,
January/December 2020, 2020-01-00, 20200101, Letnik:
29, Številka:
1
Journal Article
The Timor–Tanimbar islands of eastern Indonesia form a non-volcanic arc in front of a 7 km deep fore-arc basin that separates it from a volcanic inner arc. The Timor–Tanimbar Islands expose one of ...the youngest high
P/
T metamorphic belts in the world, providing us with an excellent opportunity to study the inception of orogenic processes, undisturbed by later tectonic events.
Structural and petrological studies of the high
P/
T metamorphic belt show that both deformation and metamorphic grade increase towards the centre of the 1 km thick crystalline belt. Kinematic indicators exhibit top-to-the-north sense of shear along the subhorizontal upper boundaries and top-to-the-south sense in the bottom boundaries of the high
P/
T metamorphic belt. Overall configuration suggests that the high
P/
T metamorphic rocks extruded as a thin sheet into a space between overlying ophiolites and underlying continental shelf sediments. Petrological study further illustrates that the central crystalline unit underwent a Barrovian-type overprint of the original high
P/
T metamorphic assemblages during wedge extrusion, and the metamorphic grade ranged from pumpellyite-actinolite to upper amphibolite facies.
Quaternary uplift, marked by elevation of recent reefs, was estimated to be about 1260 m in Timor in the west and decreases toward Tanimbar in the east. In contrast, radiometric ages for the high
P/
T metamorphic rocks suggest that the exhumation of the high
P/
T metamorphic belt started in western Timor in Late Miocene time and migrated toward the east. Thus, the tectonic evolution of this region is diachronous and youngs to the east. We conclude that the deep-seated high
P/
T metamorphic belt extrudes into shallow crustal levels as a first step, followed by doming at a later stage. The so-called ‘mountain building’ process is restricted to the second stage. We attribute this Quaternary rapid uplift to rebound of the subducting Australian continental crust beneath Timor after it achieved positive buoyancy, due to break-off of the oceanic slab fringing the continental crust. In contrast, Tanimbar in the east has not yet been affected by later doming. A wide spectrum of processes, starting from extrusion of the high
P/
T metamorphic rocks and ending with the later doming due to slab break-off, can be observed in the Timor–Tanimbar region.
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