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Volume 23 Issue 5
Oct.  2012
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Junpeng Wang, Timothy M Kusky, Ali Polat, Lu Wang, Songbai Peng, Xingfu Jiang, Hao Deng, Songjie Wang. Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China. Journal of Earth Science, 2012, 23(5): 696-704. doi: 10.1007/s12583-012-0288-8
Citation: Junpeng Wang, Timothy M Kusky, Ali Polat, Lu Wang, Songbai Peng, Xingfu Jiang, Hao Deng, Songjie Wang. Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China. Journal of Earth Science, 2012, 23(5): 696-704. doi: 10.1007/s12583-012-0288-8

Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China

doi: 10.1007/s12583-012-0288-8
Funds:

the China Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources 1212010670104

the National Natural Science Foundation of China 91014002

the National Natural Science Foundation of China 40821061

the National Natural Science Foundation of China 41272242

Ministry of Education of China B07039

Ministry of Education of China TGRC201024

More Information
  • Corresponding author: Timothy M Kusky, tkusky@gmail.com
  • Received Date: 2011-10-13
  • Accepted Date: 2011-12-23
  • Publish Date: 2012-10-01
  • The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.
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  • Banerjee, N. R., Gillis, K. M., Muehlenbachs, K., 2000. Discovery of Epidosites in a Modern Oceanic Setting, the Tonga Forearc. Geology, 28(2): 151–154 doi:  10.1130/0091-7613(2000)28<151:DOEIAM>2.0.CO;2
    Bettison-Varga, L., Varga, R. J., Schiffman, P., 1992. Relation between Ore-Forming Hydrothermal Systems and Extensional Deformation in the Solea Graben Spreading Center, Troodos Ophiolite, Cyprus. Geology, 20(11): 987–990 doi:  10.1130/0091-7613(1992)020<0987:RBOFHS>2.3.CO;2
    Deng, H., Kusky, T. M., Wang, L., et al., 2012. Discovery of a Sheeted Dike Complex in the Northern Yangtze Craton and Its Implications for Craton Evolution. Journal of Earth Science, 23(5): 676–695 doi:  10.1007/s12583-012-0287-9
    Harper, G. D., Bowman, J. R., Kuhns, R., 1988. A Field, Chemical, and Stable Isotope Study of Subseafloor Metamorphism of the Josephine Ophiolite, California-Oregon. Journal of Geophysical Research, 93(B5): 4625–4656 doi:  10.1029/JB093iB05p04625
    Jiang, X. F., Peng, S. B., Kusky, T. M., et al., 2012. Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications. Journal of Earth Science, 23(5): 705–718 doi:  10.1007/s12583-012-0289-7
    Lu, Y. F., 2004. Geokit: A Geochemical Toolkit for Microsoft Excel. Geochimica, 33(5): 459–464 (in Chinese with English Abstract) http://search.cnki.net/down/default.aspx?filename=DQHX200405003&dbcode=CJFD&year=2004&dflag=pdfdown
    Nehlig, P., Juteau, T., Bendel, V., et al., 1994. The Root Zones of Oceanic Hydrothermal Systems: Constraints from the Samail Ophiolite (Oman). Journal of Geophysical Research, 99(B3): 4703–4713 doi:  10.1029/93JB02663
    Peng, S. B., Kusky, T. M., Jiang, X. F., et al., 2012. Geology, Geochemistry, and Geochronology of the Miaowan Ophiolite, Yangtze Craton: Implications for South China's Amalgamation History with the Rodinian Supercontinent. Gondwana Research, 21(2): 577–594
    Polat, A., Appel, P. W. U., Frei, R., et al., 2007. Field and Geochemical Characteristics of the Mesoarchean (~3 075 Ma) Ivisaartoq Greenstone Belt, Southern West Greenland: Evidence for Seafloor Hydrothermal Alteration in a Supra-Subduction Oceanic Crust. Gondwana Research, 11(1–2): 69–91
    Richardson, C. J., Cann, J. R., Richards, H. G., et al., 1987. Metal-Depleted Root Zones of the Troodos Ore-Forming Hydrothermal Systems, Cyprus. Earth and Planetary Science Letters, 84(2–3): 243–253 http://www.sciencedirect.com/science?_ob=ShoppingCartURL&_method=add&_eid=1-s2.0-0012821X87900896&originContentFamily=serial&_origin=article&_ts=1425219021&md5=bba0d947e9a8826c1a9e176f314713a3
    Schiffman, P., Smith, B. M., Varga, R. J., et al., 1987. Geometry, Conditions and Timing of Off-Axis Hydrothermal Metamorphism and Ore Deposition in the Solea Graben. Nature, 325: 423–425 doi:  10.1038/325423a0
    Schiffman, P., Smith, B. M., 1988. Petrology and Oxygen Isotope Geochemistry of a Fossil Seawater Hydrothermal System within the Solea Graben, Northern Troodos Ophiolite, Cyprus. Journal of Geophysical Research, 93(B5): 4612–4624 doi:  10.1029/JB093iB05p04612
    Sun, S. S., McDonough, W. F., 1989. Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes. In: Saunders, A. D., Norry, M. J., eds., Magmatism of the Ocean Basins. Geol. Soc. Spec. Publ., London, 42: 313–345
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Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China

doi: 10.1007/s12583-012-0288-8
Funds:

the China Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources 1212010670104

the National Natural Science Foundation of China 91014002

the National Natural Science Foundation of China 40821061

the National Natural Science Foundation of China 41272242

Ministry of Education of China B07039

Ministry of Education of China TGRC201024

Abstract: The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.

Junpeng Wang, Timothy M Kusky, Ali Polat, Lu Wang, Songbai Peng, Xingfu Jiang, Hao Deng, Songjie Wang. Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China. Journal of Earth Science, 2012, 23(5): 696-704. doi: 10.1007/s12583-012-0288-8
Citation: Junpeng Wang, Timothy M Kusky, Ali Polat, Lu Wang, Songbai Peng, Xingfu Jiang, Hao Deng, Songjie Wang. Sea-Floor Metamorphism Recorded in Epidosites from the ca. 1.0 Ga Miaowan Ophiolite, Huangling Anticline, China. Journal of Earth Science, 2012, 23(5): 696-704. doi: 10.1007/s12583-012-0288-8
  • Field and petrological data suggest that epidosites form in upflow zones at the base of ore-forming hydrothermal systems in oceanic crust and lithosphere (Nehlig et al., 1994; Bettison-Varga et al., 1992; Richardson et al., 1987; Schiffman et al., 1987). Epidosites represent deep upflow zones where large volumes of hydrothermal fluilds were focused into pathways that took the fluids to the surface. Identification of epidosites can be a useful exploration tool for the location of massive sulfide deposits.

    Epidosites are well recorded in suprasubduction-zone ophiolites (Polat et al., 2007; Nehlig et al., 1994; Harper et al., 1988; Richardson et al., 1987; Schiffman et al., 1987). In this article, we report newly discovered epidosites from the Grenvillian Miaowan ophiolite (ca. 1.0 Ga, Peng et al., 2012). Field, petrographic, geochemical and oxygen isotope data are used to constrain the origin of the epidosites found in the Miaowan ophiolite. Seawater and seawater-derived hydrothermal fluids circulating through the ophiolite reacted with basaltic and diabasic rocks and were responsible for their alteration to epidosites.

  • The geological background of the Yangtze craton, Huangling anticline and Miaowan ophiolite is detailedly described in Peng et al. (2012), and Jiang et al. (2012) and shown in Fig. 1.

    Figure 1.  (a) Geological map and (b) cross section of the Miaowan ophiolite, showing the main lithotectonic units and structures; (c) structural cross section from the Proterozoic shelf sequence, through overlying flysch, wildflysh, and across the basal thrust of the Miaowan ophiolite into the deformed mafic volcanic rocks. Maps modified after Peng et al. (2012).

    The Miaowan epidosites are mainly preserved in the metamorphosed sheeted dikes, metapillow lavas and the transition zone between sheeted dikes and pillows. The metapillows have been strongly deformed and metamorphosed and are difficult to recognize, but many preserve epidote-rich cores. The pillow lavas experienced epidote-amphibolite facies metamorphism. The sheeted dike complex mainly consists of meta-diabase, meta-plagiogranite to monzogranite, meta-gabbro and ultramafic rocks, which underwent amphibolite facies metamorphism, ductile and brittle deformation, sub-sea floor metamorphism, and regional metamorphism (Deng et al., 2012).

  • The gradation from the sheeted dikes to pillows occupies a 10–30 m thick transition zone, in which the dikes pass upward into strongly flattened pillows that preserve epidote-rich cores (Figs. 2a, 3b and 3c). The dikes have an average strike of 10°–15°, significantly different from the main flattening axis of the pillows, suggesting that both units have experienced large amounts of strain and rotation, because they likely formed in a nearly perpendicular orientation.

    Figure 2.  (a) Detailed grid map showing the transition from strongly deformed almond-shaped metapillow lavas (?) to the metadiabase/plagiogranite sheeted dike complex. Location shown in Fig. 1c; (b) stereogram map (equal-area lower hemisphere projection) for foliation and lineation. Modified after Peng et al. (2012).

    Figure 3.  (a) Field photo of the outcrop of the transition zone between metadiabase dikes and metapillows; (b) amphibolite with epidosites cores; (c) amphibolite unit interpreted as deformed pillow lavas, the pistachio-green patches are epidosite alteration; (d) plagiogranite intruding into metadiabase dikes; (e) isolated epidosite lenses within metadiabase dikes; (f) late stage thin body of thin granite intruded into plagiogranite and metadiabase dikes.

    Detailed mapping of an outcrop about 10 by 12 m has been completed on the main outcrop of the transition zone from sheeted dikes to metapillows (Figs. 2a and 3a). The outcrop was divided into a 1×1 m2 grid, and each grid block was mapped individually.

    Epidote-rich cores are mainly preserved in the metapillows in the southwest part of the outcrop. Within the metapillows, epidosites are preserved as several tens of centimeters to 1 m wide and 1–3 m long zones of altered pillows. Due to the strong deformation, some of the epidosites have almond-shapes, where others form continuous and deformed bands (Figs. 3b and 3c). The selvedges of the metapillows are drawn based on the thin remnants of preserved chill margins (Fig. 2a).

    In the northeast part of the outcrop the plagiogranite and metadiabase dikes mutually intrude each other, suggesting that both rock types were em-placed simultaneously (Fig. 3d). The location of the epidosite zones is very close to the upper part of the sheeted dikes, although there are some isolated epidosites preserved in the sheeted dikes (Fig. 3e). Later granite and fine-grained dikes intrude the metadiabase dikes, plagiogranite and metapillows (Fig. 3f). Thin chill margins can be observed locally along the dike margins.

    A foliation (S1) is strongly developed within the pillows, plagiogranite and dikes. Stereograms show a clear great circle girdle distribution of S1. The strike of S1 ranges consistently from N110°E to N150°E with a preferred orientation around N125°E (Fig. 2b). S1 dips steeply to the northeast at an average angle of 70°. The S1 foliation contains a mineral lineation L1 defined by preferred orientation of hornblende that is quite difficult to observe. The L1 plunges steeply at an average orientation of 79°.

  • Epidosites in ophiolites typically occur within basaltic sheeted dikes (type A) and plagiogranites, including tonalite, trondhjemite, and quartz diorite (type B) (Banerjee et al., 2000). The epidosites in the Miaowan ophiolite are mainly preserved in the transition zone from metapillows to dikes (Fig. 2a).

  • Through their outcrop relations, the epidosites are preserved in the amphibolitic deformed metapillows. The Miaowan epidosites are characterized by assemblages of quartz+epidote (Fig. 4a). The partially altered samples are characterized by mineral assemblages of hornblende+albite+quartz+epidote (Figs. 4b and 4c). Primary clinopyroxene is replaced by hornblende±epidote. Some hornblende is altered to epidote (Fig. 4d). Based on their outcrop relations and comparison with partially altered samples from the same outcrop, we suggest that the epidosites in Miaowan ophiolite are altered from basaltic sheeted dikes and pillow lavas. The metasomatism occurred in the sea-floor alteration phase during metamorphism and gave rise to the epidosites, and are similar to other epidosites from the basaltic sheeted dikes and pillow lavas in suprasubduction-zone ophiolites. The basalts and diabase dikes were later altered to epidoteamphibolite facies mineral assemblages during regional metamorphism.

    Figure 4.  Photomicrographs of epidotized samples from Miaowan ophiolite. (a) Epidosite with epidote (Ep) and quartz (Qtz) (plane-polarized light); (b) partially epidotized basaltic amphibolite with hornblende (Hbl), epidote and quartz (cross-polarized light); (c) partially epidotized basaltic amphibolite with hornblende, albite (Ab) (cross-polarized light); (d) hornblende altered to epidote (cross-polarized light). Epidote (Ep) crystals show characteristic dispersive interference colours. All the photomicrographs are taken from probe thin sections (0.04 mm).

  • Three representative samples of epidosites were collected from amphibolitic metapillow outcrops for major and trace element, and oxygen isotope analyses.

  • Major element analyses were performed in the Comprehensive Rock and Mineral Test Center, Wuhan. Trace elements were analysed by the LA-ICP-MS lab of China University of Geosciences (CUG), Wuhan. Data analysis was performed using the geochemical software "Geokit program" (Lu, 2004).

    The epidosites show a high total REE abundance (sample C-5-1 is slightly lower) with a slight LREE enrichment. All three samples show positive Eu anomalies (Eu/Eu*=1.26–1.59) (Fig. 5). We interpret the positive Eu anomalies as the influence of the sea-floor hydrothermal alteration. Since the rocks experienced high degrees of alteration, it is difficult to assign a firm tectonic setting for the Miaowan epidosites. Based on the geochemical data of metadiabase and plagiogranite from the sheeted dikes, together with regional relationships, the Miaowan ophiolite is probably a suprasubduction zone ophiolite, formed during the subduction initiation stage (Deng et al., 2012).

    Figure 5.  Chondrite-normalized REE diagram for epidosites in Miaowan ophiolite (chondrite data after Sun and McDonough, 1989).

    The epidosites are strongly enriched in Ca and Sr and strongly depleted in Mg, K, and Na (Table 1). V is enriched, but Zn and Cu are strongly depleted, which suggests these elements may have been depleted to form ore deposits at higher structural levels. Ophiolite-hosted epidosites (Nehlig et al., 1994; Harper et al., 1988; Schiffman and Smith, 1988; Richardson et al., 1987) have markedly lower Cu abundances, and generally lower Zn contents, than their protoliths, which have been hydrothermally altered. The Miaowan ophiolite epidosites have similarly low Cu and Zn concentrations (Fig. 6), although Cu values from sample B-7-1 are much higher than those from sample C-5-1 and sample D-6-2. We interpret the higher values of Cu as a result of the miner alization at the base of ore-forming hydrothermal systems.

    Table 1.  Major and trace element analyses data

    Figure 6.  Zn vs. Cu concentration for epidosites from Miaowan, Tonga, Troodos, Josephine, and Samail ophiolites. Modified after Banerjee et al. (2000).

  • Oxygen isotope analysis was completed in the Stable Isotope Geochemistry Laboratory, Institute of Mineral and Resources, Chinese Academy of Geological Sciences, Beijing, with a precision of ±0.2‰.

    The epidosites from the Miaowan ophiolite have whole-rock δ18O values of +5.9‰, +6.2‰ and +9.7‰, which are higher than the type A epidosites from Troodos and Josephine ophiolites (+2.8‰ to +5.0‰) and higher than type A epidosites from the Tonga (+2.7‰ to +4.7‰). Fluids played an important role in the exchange equilibrium between basaltic amphibolites and epidosites during alteration. The higher δ18O values of the Miaowan epidosites are interpreted as the alteration that occurred as a result of the interaction with 18O-rich seawater or fluids. The 18O-enriched epidosite was produced during both recharge and discharge alteration involving seawater and 18O-enriched fluids.

  • Based on field relations, petrography, major and trace element analysis, and oxygen isotope data, the following conclusions are drawn for the ca. 1.0 Ga Miaowan epidosites.

    (1) The epidosites in the Miaowan ophiolite are altered from basaltic sheeted dikes and pillow lavas. The metasomatism likely occurred in the sea-floor alteration phase during metamorphism giving rise to the epidosites, and the basaltic sheeted dikes and pillow lavas in the suprasubduction-zone ophiolites were altered to epidote-amphibolite facies mineral assemblages during later regional metamorphism.

    (2) The epidosites exhibit a high total REE abundance and slight LREE enrichment with positive Eu anomalies (Eu/Eu*=1.26–1.59). The positive Eu anomalies are interpreted as a product of sea-floor hydrothermal alteration.

    (3) The Miaowan ophiolite epidosites have low Cu and Zn abundance. The higher values of Cu from sample B-7-1 are interpreted as a result of the mineralization at the base of ore-forming hydrothermal systems. It is possible that an ore deposit exists, or existed before erosion, at higher structural levels.

    (4) The higher whole-rock δ18O values (+5.9‰ to +9.7‰) for the epidosites point toward their formation in upflow zones, which can be interpreted as the result of the reaction with 18O-rich seawater or fluids during the sea-floor metamorphism beneath an ancient hydrothermal system.

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