Early Cretaceous Accretionary Complex Research Articles

Provenance analysis of Cenozoic conglomerates: Insights from Lower Cretaceous radiolarians in chert clasts from the Xigaze area, southern Tibet

Conglomerates, exposed on either side of the Yarlung Tsangpo suture zone (YTSZ) in southern Tibet, have attracted wide attention in elucidating uplift and erosion histories of the Himalayan-Tibetan orogen. However, the provenance of these conglomerates remains controversial. Although radiolarian-bearing chert clasts within these conglomerates have received little focus, identification of the radiolarian assemblages they contain could shed light on the provenance of these sedimentary units. We present the first report of Upper Jurassic to Lower Cretaceous (upper Kimmeridgian–lower Barremian) radiolarian assemblages recovered from chert clasts within the Liuqu and Gangrinboche conglomerates in the Xigaze area. To extract radiolarian fossils from independent clasts in the conglomerates, a detailed and efficient experimental process is illustrated. The assemblages are well correlated to those in the YTSZ and Tethyan Himalaya, showing typical Tethyan characteristics. The lithology of chert clasts and ages of constituent radiolarian faunas suggest that the chert clasts in Liuqu and Gangrinboche conglomerates were derived from the Bainang terrane. This interpretation implies that Early Cretaceous accretionary complexes in the YTSZ had been exposed and eroded before deposited as clasts in the Cenozoic Liuqu and Gangrinboche conglomerates.

Early cretaceous accretionary complex of the valaisan ocean, western alps?

Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor (“basalt” and “black schist” formation) and the other three units -grouped under the name of “schistes a blocs”- contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic melange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Brianconnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic melange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen “schistes a blocs” units and the absence of any Aroley limestones within the underlying tectonic melange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former melange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillere, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Brianconnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through compressional shearing tectonics, developing a “schistes a blocs” fabric and imbricating the former rift allochthons. During the Tertiary Alpine collision, Versoyen and Valaisan Trilogy units were deeply underthrusted, and then exhumed under the Subbrianconnais units - including the Petit-Saint Bernard and Arguerey Liassic calcschists, which underthrusted in turn the Brianconnais Front.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of mélange. Most mélanges are polygenetic in origin, being sedimentary or diapiric mélanges that were overprinted by tectonic deformation during subduction. Sedimentary mélanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These mélanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary mélanges from tectonic overprinting by preventing further subduction.

Origin of the pleonaste-bearing mafic-ultramafic rocks from the Armutlu peninsula, NW Turkey

Al-rich spinels were rarely reported compared to Cr-spinels, which were mostly observed in ophiolitic rocks. Pleonaste [(Mg, Fe 2+ )Al 2 O 4 ], which is an Al-rich spinel, was observed in the ophiolitic mafic-ultramafic rocks tectonically located in the Early Cretaceous accretionary complex, at the eastern part of the Armutlu peninsula, NW Turkey. The ophiolitic mafic-ultramafic rocks have cumulate character, and most of them are represented by peridotite and pyroxenite. Pleonaste was observed in pyroxenites and gabbros of the ophiolitic rocks. Pyroxenites consist mainly of clinopyroxene + orthopyroxene + amphibole ± olivine + spinel. Gabbros are composed of clinopyroxene + orthopyroxene + amphibole + plagioclase + spinel. Pleonaste in these rocks lies parallel to the magmatic layers and is distinguished by its emerald greenish color under the microscope. Pleonastes have high Al 2 O 3 (59.65-62.24 wt.%) and low Cr 2 O 3 (0.05-1.32 wt.%) contents with Mg# and Fe 3+ # ranging from 54.23 to 59.77 and 3.83 to 4.28, respectively. Petrographical observations and the pressure?temperature (P-T) pseudosection modelling suggest that pleonaste in the mafic-ultramafic rocks from the study area crystallized during magmatic processes. Presence of amphibole and Ca-rich (An % 85-88) plagioclase in these rocks suggests that the ophiolitic rocks, located in the Early Cretaceous accretionary complex at the eastern part of Armutlu peninsula, formed from an arc-related hydrous magma source.

Subduction history of the Paleo-Pacific slab beneath Eurasian continent: Mesozoic-Paleogene magmatic records in Northeast Asia

This paper presents a review on the rock associations, geochemistry, and spatial distribution of Mesozoic-Paleogene igneous rocks in Northeast Asia. The record of magmatism is used to evaluate the spatial-temporal extent and influence of multiple tectonic regimes during the Mesozoic, as well as the onset and history of Paleo-Pacific slab subduction beneath Eurasian continent. Mesozoic-Paleogene magmatism at the continental margin of Northeast Asia can be subdivided into nine stages that took place in the Early-Middle Triassic, Late Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, early Early Cretaceous, late Early Cretaceous, Late Cretaceous, and Paleogene, respectively. The Triassic magmatism is mainly composed of adakitic rocks, bimodal rocks, alkaline igneous rocks, and A-type granites and rhyolites that formed in syn-collisional to post-collisional extensional settings related to the final closure of the Paleo-Asian Ocean. However, Triassic calc-alkaline igneous rocks in the Erguna-Xing’an massifs were associated with the southward subduction of the Mongol-Okhotsk oceanic slab. A passive continental margin setting existed in Northeast Asia during the Triassic. Early Jurassic calc-alkaline igneous rocks have a geochemical affinity to arc-like magmatism, whereas coeval intracontinental magmatism is composed of bimodal igneous rocks and A-type granites. Spatial variations in the potassium contents of Early Jurassic igneous rocks from the continental margin to intracontinental region, together with the presence of an Early Jurassic accretionary complex, reveal that the onset of the Paleo-Pacific slab subduction beneath Eurasian continent occurred in the Early Jurassic. Middle Jurassic to early Early Cretaceous magmatism did not take place at the continental margin of Northeast Asia. This observation, combined with the occurrence of low-altitude biological assemblages and the age population of detrital zircons in an Early Cretaceous accretionary complex, indicates that a strike-slip tectonic regime existed between the continental margin and Paleo-Pacific slab during the Middle Jurassic to early Early Cretaceous. The widespread occurrence of late Early Cretaceous calc-alkaline igneous rocks, I-type granites, and adakitic rocks suggests low-angle subduction of the Paleo-Pacific slab beneath Eurasian continent at this time. The eastward narrowing of the distribution of igneous rocks from the Late Cretaceous to Paleogene, and the change from an intracontinental to continental margin setting, suggest the eastward movement of Eurasian continent and rollback of the Paleo-Pacific slab at this time.

砂質片岩中のジルコンの年代分布に基づく三波川帯再区分の試み

We measured the 206Pb/238U age distribution of detrital zircons in five psammitic schist samples from the Sanbagawa Belt in east-central Shikoku and the western Kii Peninsula to constrain their depositional age. The age-distribution diagrams for the five psammitic schist samples all show that detrital zircons of 100 to 90 Ma are most abundant and the age of the youngest zircon in each sample is less than 80 Ma. Considering the age of the retrogressive metamorphism of these psammitic schists, ca. 80-60 Ma, the protoliths age of the psammitic schists is constrained to 75-70 Ma, correlative to the age of the sandstone of the Middle Shimanto Belt (Yanai, 1984). A similar age-distribution has already been reported for two psammitic schist samples from the Central Unit of the Sanbagawa Belt in the Kanto Mountains (Tsutsumi et al., 2009). Thus the Sanbagawa Belt is most widely occupied by metamorphic rocks originating from rocks of the Middle Shimanto Belt. We also measured the 206Pb/238U age distribution of detrital zircons in Turonian sandstone from the Northern Shimanto Belt in the central Kii Peninsula. The age-distribution diagram shows that detrital zircons of around 128 Ma are most abundant and the age of the youngest zircon in the sample is about 100 Ma. A similar age-distribution has already been reported from a psammitic schist sample from the Southern Unit of the Sanbagawa Belt in the Kanto Mountains, overlying the Central Unit (Tsutsumi et al., 2009). The protolith age is still younger than the metamorphic age of the eclogites in central Shikoku, ca. 120-110 Ma (Okamoto et al., 2004), which occupy the uppermost portion of the Sanbagawa Belt. Although some previous studies suggested that the Sanbagawa Belt consists of metamorphosed Late Jurassic to Early Cretaceous accretionary complex, the present study shows that the belt is largely occupied by metamorphosed Late Cretaceous rocks: the Shimanto Metamorphic Rocks of Aoki et al. (2007). As a result, the Sanbagawa Belt consists of the following three units with different protolith ages: (1) Lower Unit of Shimanto Metamorphic Rocks with protoliths ages of 75-70 Ma and metamorphic ages of 70-60 Ma, (2) Upper Unit of Shimanto Metamorphic Rocks with protoliths ages of 95-85 Ma and metamorphic ages of 85-75 Ma, and (3) Sanbagawa Metamorphic Rocks (s.s.) with protoliths ages of Late Jurassic to Early Cretaceous and metamorphic ages of 120-110 Ma. The protoliths of the Upper and Lower units of the Shimanto Metamorphic Rocks are most likely rocks of the Northern Shimanto and Middle Shimanto belts, respectively.

Deformation process and kinematics of mélange in the Early Cretaceous accretionary complex of the Mino‐Tamba Belt, eastern southwest Japan

The Kanayama unit of the Mesozoic Mino‐Tamba accretionary complex in eastern southwest Japan is a thick mélange unit composed mainly of abundant muddy matrix and variously sized clasts of chert, hemipelagic siliceous mudstone, and sandstone. Two episodes of deformation formed the mélange fabrics: the first involved fragmentation of sandstone layers in response to mud injections; the second involved layer‐parallel, noncoaxial shear that resulted in the mixing of pelagic to terrigenous clasts and the formation of S‐C‐like asymmetric fabrics with scaly foliation. The S‐C fabrics exhibit a predominantly sinistral shear sense in plan view. After fold and tilt corrections, the restored slip motion inferred from the S‐C fabrics indicates a bulk top‐to‐the‐south shear. Combining the unit's terminal fossil age and radiometric age with a regional plate reconstruction model, these data suggest that the Kanayama mélange formed along a décollement during oblique subduction of the Izanagi plate in the Early Cretaceous.

Lermontovskoe Tungsten Skarn Deposit: the Oldest Mineralization in the Sikhote-Alin Orogen, Far East Russia

Abstract. Lermontovskoe tungsten skarn deposit in central Sikhote-Alin is concluded to have formed at 132 Ma in the Early Cretaceous, based on K-Ar age data for muscovite concentrates from high-grade scheelite ore and greisenized granite. Late Paleozoic limestone in Jurassic - early Early Cretaceous accretionary complexes was replaced during hydrothermal activity related to the Lermontovskoe granodiorite stock of reduced type. The ores, characterized by Mo-poor scheelite and Fe3+- poor mineral assemblages, indicate that this deposit is a reduced-type tungsten skarn (Sato, 1980, 1982), in accordance with the reduced nature of the granodiorite stock. The Lermontovskoe deposit, the oldest mineralization so far known in the Sikhote-Alin orogen, formed in the initial stage of Early Cretaceous felsic magmatism. The magmatism began shortly after the accretionary tectonics ceased, suggesting an abrupt change of subduction system. Style of the Early Cretaceous magmatism and mineralization is significantly different between central Sikhote-Alin and Northeast Japan; reduced-type and oxidized-type, respectively. The different styles may reflect different tectonic environments; compressional and extensional, respectively. These two areas, which were closer together before the opening of the Japan Sea in the Miocene, may have been juxtaposed under a transpressional tectonic regime after the magmatism.

Blueschist-Greenschist Transition in the Mt. Hibernia Schist, Union Hill, Parish of St. Thomas, Jamaica

The Mt. Hibernia Schist occurs in fault-bounded blocks on the southwestern side of the Blue Mountain Inlier in eastern Jamaica. The metamorphosed sediments and volcanic rocks are part of an Early Cretaceous accretionary complex. The metamorphism reflects a Sanbagawa facies series, wherein two facies, blueschist and greenschist, and transitional schist are recognized. The typical assemblages are: blueschist: Gln + Pmp + [Epi] + Act + Chl + Qtz + Alb + Cpx; transitional: Gln + Pmp + Epi + Act + Chl + Qtz + Alb + Cpx; greenschist: Epi + Act + Chl + Qtz + Alb + Cpx, where only traces of epidote (square brackets) occur in the blueschist assemblage. The clinopyroxene is relict in all assemblages. P-T conditions for the blueschist and transitional schist were estimated by multiple equilibria analysis involving mineral components in NCMASH. Estimated P-T conditions for the blueschist are 5.1-6.2 kbars and 290-366°C. Estimated P-T conditions for the transitional schist are 2.6-3.5 kbar and 235-240°C. Textural relationships indicate a retrograde paragenesis (blueschist to greenschist), which can be summarized by a reaction of the general form: Gln + Pmp + Cpx + Qtz = Act + Epi + Chl + Alb (1). Details of the transition are explained by three NCMASH+Fe2O3 equilibria, here generalized: Gln + Epi + Qtz + H2O = Act + Chl + Alb (2); Pmp = Epi + H2O (3); and Gln + Pmp + Qtz = Act + Epi + Chl + Alb + H2O (4). The highest P-T conditions are represented by the blueschist assemblage, Gln + Pmp + Act + Chl + Qtz + Alb + (relict Cpx). Upon decompression and cooling, conditions meet reaction (4). The equilibrium accounts for the transitional assemblage, Gln + Pmp + Epi + Act + Chl + Qtz + Alb + (Cpx), marked by the presence of epidote. Viewed as a reaction, equilibrium (4) ceases when either the glaucophane or the pumpellyite is used up, resulting in two possible assemblages—Act + Epi + Pmp + Chl + Alb + Qtz or Gln + Act + Epi + Chl + Alb + Qtz—neither of which is the greenschist assemblage. The final greenschist assemblage comes about as the result of the elimination of any remaining glaucophane or pumpelleyite by reactions (2) and (3), respectively. We propose a two-stage retrograde P-T-time path, involving (1) prolonged residence at a particular depth, consistent with a pressure of about 5 kbar for the transitional schist, followed by (2) rapid uplift and exhumation.

Tectonostratigraphic study of the Chichibu and Shimanto Belts in the Kanto Mountains, central Japan.

On the basis of 120 radiolarian biochronologic data and detailed geologic mapping, fifteen tectonostratigraphic units have been recognized in the Southern Chichibu and Northern Shimanto Belts of the Kanto Mountains, central Japan. All units are in thrust contact with each other and are numbered from north to south as Units 1 to 15. The fifteen tectonostratigraphic units are composed of accretionary deposits which were successively accreted southward during Early Jurassic to Late Cretaceous time. Each unit in the study area primarily formed a stratigraphic succession from greenstone or limestone through chert, siliceous mudstone, and mudstone to sandstone (=oceanic plate stratigraphy). The combination of those rocks is recognized not only in coherent units but in melange units. Reconstruction of the plate stratigraphy of each unit helps our understanding of the pulse of the formation of accretionary complexes in the study area. There is a lack of Early Cretaceous accretionary complexes of the Hauterivian and the Aptian to middle Albian. The drastic changes from the formation of coherent units (Middle-Late Jurassic) through the formation of melange units (latest Jurassic-earliest Cretaceous) to the accretionary hiatus (Early Cretaceous) are recognized near the boundary between the Chichibu and Shimanto Belts. The formation process for the Jurassic to Cretaceous accretionary complexes in the Kanto Mountains is therefore divided into the following three tectonic phases : the Early to middle Late Jurassic (Phase I), late Late Jurassic to Early Cretaceous (Phase II), and Late Cretaceous (Phase III). This is possibly a record of the process and the historical changes in the ancient accretionary tectonics and the relative plate motions. Phase I (the accretion of complexes in the Chichibu Belt) and Phase III (the accretion of complexes in the Shimanto Belt) may be attributed to a difference in the two subducted oceanic plates : the former was old (Izanagi Plate) while the latter was young (Kula Plate) ; moreover, the conversion of the subduction of the different oceanic plates is likely to have caused the accretionary hiatus (Phase II) in the study area.

Metamorphic Conditions in the Westphalia Schists of the Blue Mountain Inlier, Jamaica: Tectonic Implications

Two groups of metamorphic rocks—the Westphalia Schists and Mt. Hibernia Schists—occur as fault-bounded blocks on the southwestern side of the Blue Mountain Inlier in eastern Jamaica. The metamorphosed sediments and volcanic rocks are part of an Early Cretaceous accretionary complex. The Mt. Hibernia Schists reflect Sanbagawa facies-series metamorphism. Metamorphic conditions of the Westphalia Schists are less well known. In Westphalia metabasites, two metamorphic zones are recognizable—a low-T zone (I) is related to a high-T zone (II) by the reaction bio + epi = gar + hnb. The diagnostic assemblage in pelitic rocks is the same, gar + bio + chl, throughout the Westphalia Schists. These observations indicate high pressure within the amphibolite facies. In the Westphalia Schists, temperature estimates based on Fe-Mg exchange in gar-hnb are 580 to 650° C; estimates based on Fe-Mg exchange in garbio are lower—460 to 540° C. Pressure estimates for gar-hnb-plg equilibria are 6.2 to 8.6 kbar. Although temperature...

Pre‐accretionary mineralization of Japan

Abstract The metallogeny of Japan can be grouped into four environments: (1) Paleozoic‐Mesozoic stratiform Cu and Mn deposits within accretionary complexes, (2) Cretaceous‐Paleogene post‐accretionary deposits related to felsic magmatism in a continental‐margin are environment, (3) Miocene epigenetic and syngenetic deposits related to felsic magmatism during back‐arc opening, and (4) late Miocene‐Quaternary volcanogenic deposits in an island‐are environment. Group (1) deposits were a major source of Cu and Mn for the Japanese mining industry, and this style of mineralization is reviewed here. The stratiform Cu and Mn deposits were formed on the sea floor during the late Paleozoic to Mesozoic, and were subsequently accreted to active continental margins mainly in Jurassic to Cretaceous age. The Cu sulfide deposits, termed Besshi type, are classified into two subtypes: the Besshi‐subtype deposit is related to basaltic volcanism, probably at a mid‐oceanic ridge or rise; the Hitachi subtype is related to bimodal volcanism, probably in a back‐arc or continental rift. Most of the Besshisubtype deposits occur in the Sanbagawa metamorphic belt, with some occurrences in weakly metamorphosed Jurassic and Cretaceous accretionary terrains. This subtype is divided into two groups: the sediment‐barren group is hosted by basalt‐chert sequences; whereas the sedimentcovered group is hosted by basalt‐shale sequences. Both subtypes are characterized by S isotope trends similar to those of sea‐floor sulfide deposits now forming at mid‐oceanic ridges. The Hitachi‐subtype deposits occur in late Paleozoic volcanic‐sedimentary sequences and lack pelagic sediments. These deposits are characterized by association of sphalerite‐ and barite‐rich ores. The Mn deposits occur mainly in Middle Jurassic to Early Cretaceous accretionary complexes containing abundant chert beds of Triassic to Jurassic age. Their locations are well separated from those of the Cu sulfide deposits. The Mn deposits are divided into two types: the Mn type, hosted by chert, and the Fe‐Mn type, sandwiched between chert and basaltic volcanic rocks. The Mn‐type ores appear to have deposited on the deep‐sea floor further from the site of hydrothermal activity than the Fe‐Mn type. Primary Mn precipitates may have been transformed to rhodochrosite and other Mn‐minerals during diagenesis. Many of the Mn deposits were significantly metamorphosed during intrusion of Cretaceous granitoids, resulting in a very complex mineralogy.

Geometry and mechanisms of fluid flow at 15 to 45 kilometer depths in an Early Cretaceous Accretionary Complex

The Catalina Schist (southern California), contains variably metamorphosed mafic, sedimentary, and ultramafic rocks that were accreted at depths of 15 to 45 km during early Cretaceous subduction. Fluid flow in the Catalina Schist was concentrated along fractures and shear zones, as indicated by the textures and abundance of veins and evidence in melange zones for metasomatism and homogenization of stable isotope compositions. Slab‐parallel orientations of major melange zones with enhanced permeability may dictate that the majority of fluid released by devolatilization at depths >15 km in subduction zones moves updip toward the seafloor. Such transport could contribute to fluid budgets in shallower parts of accretionary wedges and facilitate large‐scale mass and energy transfer in forearc regions.