Genesis And Tectonic Significance Of The Miaowan Ophiolite Complex In The Huangling Anticline, Yangtze Craton

The Yangtze craton contains the oldest basement rocks in South China, which provide an excellent opportunity to resolve outstanding questions about the formation and evolution of Precambrian continental curst, and the assembly and break-up of the Proterozoic Rodinian supercontinent. However, there are debates about the Yangtze craton, including Meso-Neoproterozoic tectonic evolution processes, geodynamic mechanisms and relationships with the Rodinian supercontinent. Here, we report results from geological profiles, high-precision structural maps, petrography, mineralogy, geochemistry, geochronology, Lu-Hf and Sm-Nd isotopic compositions in the Miaowan Ophiolite Complex (MOC) located in the southern Huangling anticline within the Yangtze craton. We use these data to discuss the genesis and significance of the MOC, and its significance for the Meso-Neoproterozoic tectonic evolution of the Yangtze craton, the formation of the Huangling granites, and the relationship of the Yangzte craton to the assembly and break-up of the Rodinian supercontinent.The MOC mainly crops out near the Taipingxi, Dengcun and Xiaoxikou areas in Yichang city, with a WNW trend. It can be divided into two phases of magmatism:(a) the primary magmatic facies is characterized by a suite of strongly deformed rocks including serpentinized dunite and harzburgite, basalt, deformed gabbro, possible pillow lava, sheeted dike complex and metasedimentary rocks including quartzite and marble (metachert and metapelitic limestone); and (b) the late magmatic intrusion is dominated by pegmatitic-massive gabbro, massive diabase and plagiogranite, and did not experience ductile deformation process. Serpentinized dunite and harzburgite in the primary magmatic facies are exposed in the center of the complex as lenticular rock masses and slices. Chromites show disseminated, circular and layered structures, and are concentrated in the serpentinized dunite unit. Stratiform or stratoid basalt of the primary magmatic facies is preserved on the north limbs of the major ophiolitic nappe, and experienced strong ductile deformation a WNW-striking foliation associated with a strong lineation, and developed tight folds that show similar orientations of early axial planes and late penetrative cleavage. Lenticular and stratoid deformed gabbro of the primary magmatic facies shows mylonitic structure, and crops out in the basalt. Diabasic sheeted dikes of the primary magmatic facies are preserved on south limbs of the major ophiolitic nappe, and are mainly composed of diabasic dikes intruded by plagiogranitic dikes. Different dikes show either double chill margins or one-way chill margins. Possible deformed pillow lavas in the primary magmatic facies preserve epidosite-rich cores, and are intruded by late granitic veins. Banded or streaky quartzite and marble crop out in the basalt. Pegmatitic-massive gabbro and massive diabase of the late magmatic intrusion are preserved on the south limbs of the major ophiolitic nappe, and show mutually intrusive relationships.Structural features of the serpentinized dunite and harzburgite, basalt, diabasic sheeted dike and possible pillow lava of the primary magmatic facies reflect that they underwent strong deformation characterized by early WNW striking folds and shear zones. However, the pegmatitic-massive gabbro and massive diabase of the late magmatic intrusion show weaker deformation. The primary magmatic facies and late magmatic intrusion show high-angle fault contact relationships in some places, but some outcrops show that pegmatitic gabbro intrudes serpentinized harzburgite along fractures. Results of the high-precision structural mapping and foliation measurements (n=108) and lineation (n=19) of the basalt, diabasic sheeted dikes and possible pillow lavas areas of the primary magmatic facies show that the penetrative foliation and lineation dip steeply to the WNW. The deformed orientation of the primary magmatic facies is consistent the foliation and high-angle thrust faults of the basal thrust belt, and the distribution and kinematic features of the entire MOC.Petrographic and mineralogical features of the serpentinized dunite and harzburgite from the primary magmatic facies show that they experienced strong metamorphism. Most olivine grains have been replaced by serpentine and talc, with remnant olivine grains surrounded by the alteration minerals. Pyroxene grains are typically replaced by tremolite and bastite. The olivine in serpentinized dunite of the primary magmatic facies is magnesium-rich (Fo=91.07-91.85). The ultramafic rocks of the primary magmatic facies include several podiform chromite bodies with the core-rim structure. The textures of the chromite include disseminated chromite, nodules, and are locally associated with multi-ring orbicules of serpentinite and magnesite. Electron probe micro-analyser (EPMA) results show that the core of the chromite has high contents of chromium, magnesium and aluminium in comparison with the high content of iron, titanium and manganese of the rim. The Fo values (26.71-41.59) of the core have a negative correlation with the Cr# ratios (0.61～0.67), however, no significant correlation was observed between the Fo values (98.11～99.09) and Cr# ratios (0.84-0.96) of the rim. In the Cr#-TiO2 and Fe/(Cr+Al+Fe) diagrams, EPMA data of the chromite core were all plotted into the transition zone between mid-ocean ridge and island arc settings. Pyroxene grains of the basalt and deformed gabbro of the primary magmatic facies, pegmatitic-massive gabbro and massive diabase of the late magmatic intrusion are typically replaced by magnesio-hornblende. Plagioclase of the basalt and gabbro are labradorite and bytownite, which belong to the basic plagioclase series, however, plagioclase of the diabase is andesine. Lenticular and stratoid metachert-pelitic limestone in the primary magmatic facies consists of quartz, calcite, garnet, diopside, hornblende and biotite in which the diopside grains coexist with hornblende. The evidence indicates that the metamorphic grade of these rocks reached upper amphibolite facies.Deformed gabbro samples in the primary magmatic facies have yielded weighted mean 207Pb/206Pb ages of 1118±24 Ma and 1096±32 Ma, respectively, and the ultramafic rocks, deformed gabbros and basalts of the primary magmatic facies collectively yield a Sm-Nd errorchron average age of 1135±54 Ma. The whole-rock errorchron and zircon U-Pb ages agree within analytical errors, indicating that the primary magmatic facies formed between 1135 and 1096 Ma. Massive gabbros, pegmatitic gabbros and massive diabase in the late magmatic intrusion have yielded weighted mean 207Pb/206Pb ages of 1002±19～999±17 Ma,974±11～971±16 Ma and 978±12 Ma, respectively, and plagiogranite intruded the diabase of the sheeted dikes, yielding a weighted mean 207Pb/206Pb age of 983±7 Ma. Regression of the massive-pegmatitic gabbros and massive diabases in the late magmatic intrusion yields an errorchron age of 1007±62 Ma. The whole-rock errorchron and zircon U-Pb ages agree within analytical errors, indicating that the late magmatic intrusion formed between 1007 and 971 Ma. These ages show that the MOC experienced different tecttono-magmatic evolution stages, the primary magmatic facies formed in the early stage, ranging from 1135～1096 Ma, and the late magmatic intrusion formed in the late stage between 1007 and 971 Ma.Magmatic-type detrital zircon cores from the lenticular and stratoid quarizite and marble yield ages of 967 to 1105 Ma(mean = 1009±32 Ma) and 1011 to 1095 Ma(mean = 1054±13 Ma) ages, respectively, representing the maximum depositional time for them. The metamorphic age of 1052 ± 18 Ma obtained from the marble is close to the mean maximum depositional age, indicating that these measured zircon grains experienced incomplete recrystallization. The remaining two metamorphic ages (941 ± 12 Ma and 936 ± 18 Ma) suggest that the peak metamorphism occurred during the early Neoproterozoic collision process in the Yangtze craton. These metamorphic ages are similar to those of the Grenvillian-aged collisional events recorded in orogenic belts worldwide. Weak deformed granitic pebbles collected from the wild-flysch sequence and granitic vein taken from the possible pillow lava area in the MOC have yielded weighted mean 206Pb/238U ages between 870 and 810 Ma, representing the magmatic events for them, which are similar with the formation age of the Neoproterozoic Huangling granites.Geochemical data of the serpentinized dunite and harzburgite in the primary magmatic facies show that these rocks are depleted in Ti and Al, but have relative high-contents of REE. The REE distribution patterns of the meta-ultramafic rocks can be divided into two kinds. The first kind show depleted LREE patterns, and the second type shows a slight loss of middle REE forming a U-shaped plot, which is similar to the characteristics of metamorphic ophiolitic mantle peridotite. The REE distribution patterns of the basalt and deformed gabbro in the primary magmatic facies show flat to very slightly depleted LREE patterns, depleted in Th, Ti, Zr and Hf, but enriched in Nb. On Ta/Yb-Nb/Yb and La/Yb-Nb/Yb diagrams, geochemical data of basalt and deformed gabbro samples in the primary magmatic facies fall into the field between N-MORB and E-MORB, indicating that the formation process of these rocks is controlled by the magmatic source. The REE distribution patterns of the massive-pegmatitic gabbro and massive diabase samples show slightly enriched LREE patterns, depletion in Nb but enrichment of Th. On a Ta/Yb-Nb/Yb discrimination diagram, these samples plot into the area of the oceanic mantle array, but results of the La/Yb-Nb/Yb diagram reveal that the massive-pegmatitic gabbro and massive diabase samples fall into the area away from the oceanic mantle array. The evidence indicates that the formation process of these rocks in the late magmatic intrusion is controlled by magmatic source and tectonic metamorphic facies in the late stage (melt/rock reaction). The geochemical features of the primary magmatic facies and late magmatic intrusion indicate that they may have the different formation processes.Initial εNd values of ultramafic rocks, basalts and deformed gabbros in the primary magmatic facies range from +5.7 to 7.6 (Mean=+7.0), and the εHf(t) values of the deformed gabbro are of +12.8 to+17.4, displaying an average value of 13.91 ± 0.82, corresponding to Hf depleted mantle model ages (Tdm1) are 1131～931 Ma (mean=1069±44 Ma). Initial εNd values of massive-pegmatitic gabbro and massive diabase of the late magmatic intrusion range from +6.0 to +7.2 (mean=+6.7), and εHf (t) values are +11.91～+13.82 (mean=+12.69 ± 0.45), +11.95～+13.06 (mean=12.63±0.41) and +11.73～+13.54 (mean=12.71±0.47), respectively, corresponding to Hf depleted mantle model ages (Tdmi) of 1061～987 Ma (mean=1032±28 Ma), 1035～979 Ma (mean=1008±28 Ma) and 1049～974 Ma (Mean=1008±28 Ma), respectively. Initial εHf (t) values of the plagiogranite are of +13.24～+16.96 (mean=+14.32±0.64), corresponding to Hf depleted mantle model ages (Tdm1) are between 1051 and 898 Ma (mean=999±31 Ma). The large positive εNd values for the ultramafic rocks (serpentinized dunite and harzburgite), basalts, deformed gabbros in the primary magmatic facies, and pegmatitic-massive gabbros and massive diabases in the late magmatic intrusion are collectively consistent with depleted mantle sources. Moreover, there is a close match between the zircon U-Pb ages and Hf depleted mantle model ages for the mafic rocks of the MOC, and initial Hf values from zircons are also consistent with a strongly depleted mantle source. These data suggest that the northern Yangtze craton experienced the process for the growth and rapid reworking of juvenile crust during the Meso-Neoproterozoic magmatism, and for significant transport of materials from the depleted mantle to the continental crust.Lu-Hf isotopic compositions of zircon from the layered-banded quartzite and marble samples can be divided into two types. The first kind has small positive εHt (t) values ranging from +2.61 to +7.41 (mean=5.90±2.90), which are plot in the transition zone between depleted mantle and continental crust, and Hf depleted mantle model ages varies from 1456 to 1254 Ma (mean=1345±130 Ma) The second type has large εHf (t) values are of +9.2～+14.08 (mean=11.42+0.93), which fall into the depleted mantle array, and Hf depleted mantle model ages range from 1215-984 Ma (mean=1130±40 Ma). On Th-Sc-Zr/10 and La-Th-Sc diagrams, majority of the quartzite and limestone samples in the MOC fall into oceanic island arc and continental island arc fields, indicating that their original source may have been an oceanic island arc setting gradually transformed into a continental island arc environment, and the latter played an increasingly important role in the depositional process. Accordingly, it is proposed that the protolith of the quartzite and marble were deposited on the oceanic crust in a forearc setting where the volcanic clastic materials eroded from the juvenile crust and island arc are involved in the deposition, which carry the amounts of detritus zircons. Therefore, the metasedimentary rocks share close genetic relationships with the MOC. Additionally, the stratigraphic sequence of basalt to quartzite to marble is not consistent with typical ocean plate stratigraphy in which limestone would be deposited at the ridge, then overlain by deep-sea cherts as the oceanic lithosphere cooled and subsided below the CCCD as it moved away from the ridge. This sequence is however consistent with a fore-arc setting that became shallower with time. The sandstone samples in the MOC fall in the continental island arc field, suggesting that the source was mainly a continental arc. From the cross section of the basal thrust belt of the MOC, we can observe that the sandstone is located south of the basal thrust and its tectonic setting is in a fore-deep basin. The geochemical data suggest that the sandstones have composite immature sources and formed in tectonically active settings. In these regards, it is proposed that the sandstone in the MOC may mainly have been eroded from overriding accretionary wedge of the thrust front related to emplacement of the MOC over the passive margin.Lu-Hf isotopic compositions of zircons from the granitic pebbles taken from the wild-flysch sequence of the basal thrust belt in the MOC can be divided into two types. The first kind has negative eHf (t) values ranging from -24.99 to -35.00 (mean=-29.20±2.80), Hf depleted mantle model ages (T<dm2) are varies from 2989～3528 Ma (mean=3219±150 Ma), which are consistent with the Hf composition results of the Huangling granitoids. The second type has large positive εHf (t) values of+9.53～+10.03 (mean=+9.90±1.30), Hf depleted mantle model ages (Tdm2) range from 1077～1104 Ma (mean=1091±69 Ma), which are similar with the Hf composition of the mafic rocks and plagiogranite in the MOC. The evidence indicates that the Neoproterozoic granites were mainly formed by the anatexis of the Archean continental crust, but also suffered reworking of juvenile crust in the late Mesoproterozoic.Although the results of whole-rock Sm-Nd and zircon Lu-Hf isotopic compositions of the primary magmatic facies and late magmatic intrusion indicate that they are derived from the depleted mantle, the geochronological data suggest that the magmatism event of the primary magmatic facies ca. 120 Ma earlier than the late magmatic intrusion, with distinct geochemical features. The evidence implies that the primary magmatic facies and late magmatic intrusion sformed in different tectono-magmatic stages. Therefore, a geodynamic model is proposed for the evolution of the MOC:(1) during the late Mesoproterozoic (1130～1100 Ma), an oceanic basin located along the northern margin of the Yangtze craton was undergoing northeastward subduction and generated the Shennongjia arc volcanism, which is composed of calc-alkaline basalts and tholeiitic andesites. Formation of the fore-arc basin along the southeastern margin of the Shennongjia arc, which is represented by the primary magmatic facies consists of ultramafic rocks, deformed gabbro, basalt, sheeted dike and possible pillow lava; (2) Shennongjia arc collided with the Yangtze craton at ca.1000 Ma, and the primary magmatic facies was thrust onto the Yangtze craton and imbricated with the continental shelf sequence; (3) back-arc basin generated southwestward subdcuting zone, forming the Xixiang arc composed of basaltic and andesitic lavas, meanwhile, partial melting of subducted slab formed the late magmatic intrusion consists of weak deformed pegmatitic-massive gabbro and massive diabase at 1000-970 Ma; (4) metamorphic ages 941±12 Ma and 936±18 Ma for the metachert and metapelitic limestone in the primary magmatic facies, respectively, suggest that the Yangtze craton-Shennongjia arc collided with Australia plate during the early Neoproterozoic, forming the entire Miaowan ophiolitic nappe; (5) during the 870-810 Ma, post-orogenic collapse caused by continental crustal thickening, forming the Neoproterozoic Huangling granitoids and coeval plutons.