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

INTRODUCTION

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.

GEOLOGIC SETTING

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).

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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).

STRUCTURAL ANALYSIS

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).

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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.

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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°.

TYPES OF EPIDOSITES

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).

SAMPLE DESCRIPTIONS AND ETROGRAPHY

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).

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GEOCHEMISTRY

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

Major and Trace Elements

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).

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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

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

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Oxygen Isotopes

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.

CONCLUSIONS

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.