Fabric analyses based on the anisotropy of magnetic susceptibility (AMS) provide a criterion with which to distinguish tectonic from sedimentary mélanges in the Tertiary Shimanto accretionary ...complex, Yakushima Island, SW Japan. Sedimentary mélanges are characterized by fabric overprinting associated with compaction and diagenesis. The magnetic ellipsoids for such mélanges represent a cleavage/bedding-parallel magnetic foliation, similar to that obtained for coherent sedimentary layers in nearby rocks. The tectonic mélanges and tectonically formed duplex structure are characterized by grain rotation, whereby the grain short-axis is rotated about axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, perpendicular to the shear direction. The AMS data obtained for tectonic mélanges and duplex structure show a less intense magnetic foliation compared with the sedimentary mélanges, with Kmax (the maximum magnetic susceptibility) axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, and Kmin (the minimum magnetic susceptibility) axes showing a girdle distribution about Kmax. These contrasting paths of fabric development may reflect different stages of mélange formation during the overall history of sediment deposition, burial, accretion, and uplift. Sedimentary mélanges are likely to be strongly compacted, given that they formed soon after sedimentation. In contrast, tectonic mélanges and duplex structure contain a shear-induced fabric because they develop at a relatively late stage, once the sediments have been sufficiently compacted for such a fabric to develop, possibly at levels near the maximum burial depth.
► Fabric analyses of mélanges provided a criterion to distinguish tectonic from sedimentary origins. ► Sedimentary mélanges are characterized by fabric overprinting associated with compaction and diagenesis. ► Tectonic mélanges are overprinted on compaction-related fabrics or shale fabrics, accompanied by composite shear planes. ► Tectonic mélanges show a less intense magnetic foliation compared with the sedimentary mélanges.
The last interglacial period, from 130 to 117ka, was as warm as or warmer than the present, and thus it is relevant for our understanding of the possible effects of recent global warming. In this ...study, we used radiolarian assemblages in a sediment core to reconstruct environmental fluctuations in the northwestern Pacific Ocean during the last interglacial period. The sediment core (MD012421) was collected where the warm Kuroshio Current meets the cold Oyashio Current. We used the ratio of warm-water to the total of warm- and cool-water radiolarian species (Tr) as an indicator of the dominant surface water masses (relative importance of the warm Kuroshio and cool Oyashio Currents), and we used the relative abundance of Cycladophora davisiana as an indicator of cold, oxygen-rich intermediate waters related to seasonal sea ice formation at high latitudes.
Tr results indicated that a transitional surface water mass off Kashima was replaced by a subtropical water mass between 131ka (late Marine Isotope Stage MIS 6) and 125ka (last interglacial period; MIS 5e). Furthermore, the relative abundance of C. davisiana changed from 19.6% at 133ka (late MIS 6) to ~0% at 126ka (MIS 5e); this change suggests a reduction of cold, oxygen-rich waters at intermediate depths off Kashima in association with weakened ventilation at high latitudes in the North Pacific. As this reduction took place ~1kyr earlier than the shift of surface water masses at the core site, we inferred that changes of insolation (the main driver of glacial–interglacial cycles) controlling the variation of sea ice formation at high latitudes occurred earlier than the lower latitude change in the atmospheric pressure pattern controlling the surface water shifts. High-resolution time-series analysis indicated that fluctuations in the surface water mass occurred cyclically, with a periodicity of approximately 500years during the second half of the last interglacial period. This periodicity may be attributable to fluctuations in solar radiation.
•Transitional surface water replaced by subtropical water during MIS 6 and 5e.•Cold, oxygen-rich intermediate waters reduced during MIS 5e.•Changes in solar radiation caused an ~500-yearcycle in surface water properties.
Exposure of the
ca. 6
Ma Taitao ophiolite, Chile, located ∼50
km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the ...depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ
18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ
18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by
87Sr/
86Sr and
143Nd/
144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial
187Os/
188Os ratios that range widely from 0.1168 to 0.1288 (
γ
Os
=
−8.0 to +1.1), averaging 0.1239 (
γ
Os
=
−2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg
# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6
Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.
In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ
18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial
187Os/
188Os ratios, calculated for 6
Ma, that range from 0.126 (
γ
Os
=
−1) to as high as 0.561 (
γ
Os
=
+342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in
187Os/
188Os between MORB and abyssal peridotites.
The collective results of this study suggest that this >500
km
3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6
Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1
Ga period was remarkably inefficient, at least with regard to Os isotopes.
The Taitao granites are distributed around the Late Miocene Taitao ophiolite (5.66
±
0.33 Ma to 5.19
±
0.15 Ma) exposed at the western tip of the Taitao peninsula, southern Chile, ~
50 km southeast ...from the present day Chile triple junction. In this paper, we report sensitive high mass-resolution ion microprobe (SHRIMP) U–Pb ages for the Taitao granites to elucidate the temporal relationship between the ophiolite and granites, and discuss the origin of the granitic melts. Five intrusive bodies of the Taitao granites have U–Pb ages ranging from 5.70
±
0.25 Ma (Tres Montes pluton in southeast) to 3.92
±
0.07 Ma (Cabo Raper pluton in southwest). The Estero Cono, Seno Hoppner and Bahia Barrientos intrusions that fringe eastern margin of the ophiolite have U–Pb ages ranging from 5.17
±
0.09 Ma to 4.88
±
0.3 Ma. Recycled zircon cores are common only in the Tres Montes pluton. Our data indicate that the generation of the granitic melts started in the Tres Montes area when a short segment of the Chile ridge system started to subduct ca. 6 Ma ago. This magmatism involved contamination with sediments/basement rocks. A part of the subducting ridge center was emplaced to form the present Taitao ophiolite at ~
5.6 Ma. Generation of granitic melts continued as the spreading center of the same ridge segment subducted, due perhaps to partial melting of the ophiolite and/or oceanic crust enhanced by heat from upwelling mantle beneath the ridge. Granitic magmas with various compositions developed during subduction of the ridge. Emplacement of the ophiolite and formation of continental crust took place almost simultaneously.
Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on ...a 15 Ma old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end‐member, superfast‐spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo single‐domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike‐normal orientation of the minimum AMS axes (Kmin) of prolate AMS ellipsoids mean that the long axis (Kmax) can be used to infer magma flow directions. Subvertical Kmin orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of Kmin axes, orientation of Kmax axes indicate a rather constant subhorizontal paleo‐flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.
Key Points
Melts were transported laterally during intrusion at EPR
Clustering of Kmax axes and prolate fabrics were used to obtain melt directions
Kmin axes indicate that secondary processes affected the petrofabrics
Abstract
Several swarms of parallel E–W dikes cuts late Neoproterozoic- to early Paleozoic (540 Ma) basement in the Biarjmand area, NE Iran. The microgabbroic to doleritic dike are several hundred ...meters long and 0.5 to 3 meters wide. U–Pb dating of their zircon grains yields a crystallization age of 46.0 ± 7.1 Ma for the dike swarms. The dikes have SiO2 contents of 46.0–49.3 wt%, and Fe2O3 contents of 9.80–14.8 wt% with variable MgO (4.92–9.16 wt%), TiO2 (1.47–2.65 wt%), and K2O contents (0.33–1.13 wt%). The dikes have low contents of high field strength elements (HFSEs), and have some similarities to transitional mid-ocean ridge basalts (T-MORB) based on their chemical composition. Positive ${\varepsilon}_{Nd}(t)$ values of +5.2 to +6.0 for the dike samples imply that the basaltic magma was generated from depleted lithospheric mantle. After the closure of the Sabzevar ocean and obduction of the large mantle peridotite body over the continental crust in the late Cretaceous-Paleocene, the gravitational instability in the central Iran/Eurasia plate collision zone triggered exhumation of old basement coeval with injection of the mafic magma. During exhumation, mylonitic deformation with brittle-ductile structures affected the basement metagranite and the host rocks. The metagranite and dike swarms cut the Cretaceous Sabzevar ophiolite, and the disparting of ophiolite members probably occurred during basement exhumation in the Cenozoic. The similar ages of the mafic dike swarms and other Eocene magmatic rocks of the Urumiah (Urmia) Dokhtar magmatic arc (UDMA), western Iran, is consistent with a rapid extensional regime over the Sabzevar suture zone on an earlier collision zone in NE Iran. This process provided a suitable setting for the exhumation of the old basement, the emplacement of the parallel dike swarm, and the development of shallow basins in this area.
Geochemical compositions are reported for Late Miocene to Pliocene granitoids from the Taitao Peninsula near the Chile ridge subduction zone. Major element compositions of Taitao granitoids show a ...resemblance with those of TTG suites. However, trace element compositions are characterized by low Sr (50-300 ppm), moderately high Y (10-45 ppm) and Yb concentrations (1-5 ppm), and low Sr/Y and La/Yb ratios compared with those of typical adakites, which are presumably produced by melting of young and hot oceanic crust under eclogite to garnet amphibolite conditions. Instead, trace element composition of the Taitao granitoids resembles that of typical calc-alkaline arc magmas. Based on trace element compositions, together with tectonic constraints, we infer that the Taitao granitoids were generated by partial melting of the subducted oceanic crust in garnet-free amphibolite conditions at depths shallower than 30 km. Our results indicate that slab-melting-related magmas do not necessarily show a HREE-depleted signature, which was used as evidence for slab-melting for granitic rocks of the TTG suites.
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