Identification And Tectonic Implication Of The Neoarchean Zanhuang Melange In The Central Orogenic Belt, North China Craton

Sutures mark places where oceans have closed, and two once widely separated terranes (arcs, continents, plateaux, etc) have collided, thus are remnants of ancient subduction zones. Ancient sutures, especially Archean sutures, have complicated internal structural relationships and are often overprinted by later geological events, leading to the difficulty of recognition. Suture zones are typically characterized by structurally complex rock units such as melanges and ophiolites that were scraped off the intervening oceanic substratum during convergence and collision of two terranes. Melanges are one of the hallmark units of accretionary complexes at convergent plate margins, and typically record the evolution history of accretionary tectonics along convergent margins, and mark the suture zones which formed when accretionary orogens convert to collisional orogens. Therefore, detailed multidisciplinary study focusing on the structural and metamorphic history of ancient melanges which includes field mapping, structural and kinematic analyses, geochronology and geochemistry can yield important information on the early history of orogenic belts.The North China Craton records important information for the early evolution of the earth’s crust, and is one of the most important natural laboratories to study the evolution of ancient cratons. Many researchers have studied for decades on the evolution of the ancient crust, splitting and collision processes in Proterozoic, and destruction in Mesozoic and Cenozoic of the North China Craton, and have made important research progresses. However, the problem related to the formation of the Precambrian basement of the North China Craton and its tectonic evolution has been controversial among domestic and international geological communities. Many scientific issues remain unsolved, including the formation mechanism, nature and timing of amalgamation of various blocks, and subduction polarity. The Zanhuang massif located on the eastern margin of the central and southern section of the Central Orogenic Belt preserves geological records of an Archean arc/continent collision. The Zanhuang massif is one of the important areas to study the collisional orogenesis between the Eastern and Western Blocks and is essential to fully understand the formation history of the basement of the North China Craton. Based on detailed field investigation, the author recognized a well preserved tectonic accumulated rock sequence (TARS) that has similar characteristics with other typical melanges worldwide. Therefore, based on high-precision (1:20,1:40) field mapping of main outcrops, the author takes the TARS as an object of this study and designs to 1) prove the TARS as a tectonic melange belt which is named "Zanhuang melange",2) make detailed analysis of its internal litho-structural assembly, fabrics and kinematics in order to decode the structural style and restore its formation history,3) combine petrology, geochemistry, Sm-Nd isotopic geochemistry and geochronological studies of the Wangjiazhuang granite and undeformed pegmatite which intrude the "Zanhuang melange" to restrict the formation age of the Zanhuang melange,4) provide further constraints on the formation mechanism, nature and timing of amalgamation of various blocks, and subduction polarity of the North China Craton, and 5) enrich and consummate Neoarchean tectonic evolutionary model of the North China Craton in the end.Detailed structural analysis and large scale litho-structural mapping show that the TARS is composed of a structurally complex tectonic mixture of metapelites, metapsammites, marble, metalimestone, and tectonic blocks of ultramafites and metagabbroic rocks, metabasites that locally include relict pillow lavas, and TTG gneisses. Different kinds of blocks including metamafic-ultramafic rocks, metabasites, gneisses and marbles are distributed chaotically in metapelitic and metapsammitic matrixes. All the units in the TARS have been intruded by mafic dikes that were subsequently deformed, and are now preserved as garnet-amphibolite boudins. The TARS preserves the typical "block in matrix" structure, with different rock types and origins of the blocks and matrixes. The relict pillow basalts with epidosite cores are distributed in a strongly deformed amphibolite and metapelitic matrix. However, the mafic-ultramafic blocks are mainly dispersed in metapelites. In addition, small marble blocks can be recognized in micaschist matrixes, showing strong deformtion and repetition between the mable and micaschist.The litho-tectonic units of the TARS have undergone strong deformation and accumulation and are imbricated by numerous northwest-dipping thrusts. Folds and faults are well developed in the TARS. Folds are widespread in chert, metapelite, gneiss and quartzite in the TARS, with rare sheath folds preserved in the chert fragments within the micaschist. Asymmetric extensional structures are abundant in the TARS, including scaly foliation, asymmetric tails around blocks or the shapes of blocks. The structural elements show consistent trends with regional structure that the TTG gneisses of the Western Zanhuang Domain are thrust to the southeast upon the TARS. The highly sheared and deformed TARS is thrust to the southeast upon the marble-siliciclastic unit which has been deposited along the western margin of the Eastern Block. Most kinematic indicators from the TARS show southeast-directed thrust sense of motion.The TARS has a northeast-southwest trending with the internal fabric showing overall northeast-southwest striking. The field relationships show that the TARS was intruded by the Wangjiazhuang granite and crosscut by the undeformed pegmatite. Most of the foliations in the TARS strike northeast and dip steeply northwest, but the strike of the foliation is rotated into near-parallelism with the Wangjiazhuang pluton margins. In the northeast and southwest parts, the late Wangjiazhuang granite truncates the early foliation in the TARS, indicating that the TARS formed before the intrusion of the Wangjiazhuang granite. The undeformed pegmatite crosscuts many sections of the TARS including mafic boudin, marble and gneisses, suggesting that the TARS formed prior to the formation of the undeformed pegmatite.The Wangjiazhuang grante located in the northern part of the study area intrudes the TARS. The main minerals include microcline (55-60%), quartz (25-30%), biotite (5-10%), muscovite (<5%) and minor magnetite, apatite and zircon. The Wangjiazhuang granite has a relatively narrow range of geochemical compositions, with high SiO2, Al2O3, MgO, Fe2O3T and CaO. These rocks are relatively rich in alkalis (K2O+Na2O), and have low contents of TiO2. The aluminum saturation index A/CNK is between 1.32 and 1.41, with an average value of 1.37, showing peraluminous characteristics. The geochemical characteristics of the Wangjiazhuang granite are comparable to those of the shoshonitic series and show that the Wangjiazhuang granite belongs to A-type granite. The Wangjiazhuang granite has a large variation and enrichment in trace elements. Total REE contents of the Wangjiazhuang granite are high, and show wide variations (∑REE=282.1-587.1 ppm, average value= 441.8 ppm) with strong negative Eu anomalies (Eu/Eu*=0.25-0.31). The Wangjiazhuang granite is highly enriched in LREEs ((La/Sm)N=4.4-4.9; (La/Yb)N=8.5=31.4) and has relatively flat HREE ((Gd/Yb)N= 1.3-3.3) patterns. On the primitive mantle-normalized spidergram, the Wangjiazhuang granite has negative anomalies in Ba, Sr, P and Ti. Saturation temperature of zircons picked up from the Wangjiazhuang granite is between 806 and 861 ℃ with an average value of 836 ℃, which is interpreted to represent the formation temperature of the Wangjiazhuang granite. The main minerals of the undeformed pegmatite inlcude microcline (50-55%), plagioclase (5-10%), quartz (15-20%) and tourmaline (10-15%).Whole-rock Sm-Nd isotopic geochemistry results of the Wangjiazhuang granite show that the low sNd (t) values (+0.12 to+1.13) are consistent with contamination by ancient crust. The one stage Nd isotope model ages (2750-2863 Ma) and two stage Nd isotope model ages (2784-2869 Ma) are older than the formation age of the Wangjiazhuang granite (ca.2.5 Ga) and similar to the age of the TTG gneisses (ca.2.7 Ga) in the study area. The Wangjiazhuang granite has a relatively narrow range of fSm/Nd values (-0.41 to -0.47), indicating that the fractionation of Sm and Nd in the source region is unobvious, and the Nd isotope model ages can be used to represent the formation age of the source region.LA-ICPMS U-Pb data of the zircons from the Wangjiazhuang granite and pegmatite show that the formation ages of three granite samples are 2517 ± 20 Ma (NSWD= 2.7),2506 ± 10 Ma (NSWD= 0.56) and 2513 ± 13 Ma (NSWD= 1.9), while the formation age of one undeformed pegmatite is 2539 ± 44 Ma (NSWD= 1.8). The formation ages of all the samples are close to 2.5 Ga.Therefore, based on detailed field mapping, petrological geochemical, Sm-Nd isotopic and geochronologcial studies of the TARS, the following understandings are obtained:1) We have confirmed that the TARS is a tectonic melange which is named "Zanhuang melange". Melanges occur widely in collisional and accretionary orogenic belts around the world and represent mappable geological units consisting of blocks with different ages and origins, and are commonly embedded in an argillitic, sandy, or serpentinite matrix showing high stratal disruption and a chaotic internal structure. Melanges have typical "block in matrix" structure and show strong deformation and metamorphism. The TARS in this study is a polygenetic complex consisting of varieties of rocks from different origins with a complicated tectonic history, fits the classical criteria for defining melange.2) The formation age of the Zanhuang melange is prior to Neoarchean (ca.2.5 Ga). Based on field observations that the Zanhuang melange was intruded by the Wangjiazhuang granite and crosscut by the undeformed pegmatite, the formation ages of the Wangjiazhuang granite and pegmatite (ca.2.5 Ga) provide the minimum age for the formation of the Zanhuang melange.3) The Zanhuang melange marks a suture zone of a subduction-collision between an arc terrane in the Central Orogenic Belt and the Eastern Block of the North China Craton, and records key information of the subduction-accretion process. Previous geochemical and geochronological studies show that the Eastern Zanhuang Domian consists of Archean TTG gneiss belonging to the Eastern Block of the North China Craton, and represents an old continental crust. The Western Zanhuang Domain consists of tonalitic gneisses with ages of 2.7 Ga, and is spatially and temporally correlated with the Fuping terrane. The marble-siliciclastic unit deposited along the western margin of the Eastern Zanhuang Domain (Eastern Block) is a sequence of metasandstone, marbles, and metapelites, grading up into a matagraywacke-pelite unit, which is interpreted as a passive margin sequence to foreland basin flysch sequence. Therefore, the Zanhuang melange separates the arc terrane in the Central Orogenic Belt and the passive margin and foreland basin sequence along the western margin of Eastern Block. Field observations and geochemical studies on the exotic mafic-ultramafic-metabasaltic blocks show that they could be residue of intra-oceanic arc-forearc ophiolite and were later incorporated into the melange during an arc-continent collision. Accordingly, we interpret the Zanhuang melange to be an Archean suture zone between the arc terrane in the Central Orogenic Belt and Eastern Continental Block of the North China Craton. This suture zone is a result of an Archean arc-continent collision, rather than a Paleoproterozoic continent-continent collision according to traditional view point. Tectonic models concerning on the evolution of the Central Orogenic Belt are varied, but include models that favor collision at 2.5 Ga,2.1 Ga, and 1.8 Ga. This work shows, from field structural relationships and geochronology, that accretionary event and collision which gave rise to the melange must have occurred prior to 2.5 Ga, consistent with late Archean suturing of the western margin of the Eastern Block with an arc terrane (Fuping terrane) during an arc-continent collision. The litho-tectonic units of the Zanhuang melange experience intensive deformation, transformation and accumulation, with overall characteristics of dipping to northwest thrusting and imbrication, which is consistent with a tectonic setting of accretionary wedge. The internal deformation characteristics of the Zanhuang massif are a result of the collision between the Fuping terrane in the Central Orogenic Belt and the Eastern Block of the North China Craton.4) The Central Orogenic Belt and the Eastern Block of the North China Craton both experienced one stage of arc-related magma intrusion event during Neoarchean. Petrology and geochemistry of the Wangjiazhuang granite show that the Wangjiazhuang granite belongs to A-type granite, and was formed in an arc tectonic setting related to subduction. The Sm-Nd isotopic data show that the Wangjiazhuang granite has similar Nd model ages with the widespread 2.7 Ga TTG gneisses in the study area, suggesting that the Wangjiazhuang granite may have resulted from partial melting of the ca.2.7 Ga TTG gneisses in the Zanhuang massif. Based on a comparison between the Wangjiazhuang granite and other granitic rocks within the North China Craton with similar age (ca.2.5 Ga), geochemical characteristics, and petrogenesis (A-type granite), it turns out that the ca.2.5 Ga granitic magmatic intrusions are not only distributed in the Zanhuang massif, but are throughout the Central Orogenic Belt and Eastern Block of the North China Craton. These rocks have geochemical characteristics of potassic A-type granite, and are formed in an subduction-related arc tectonic setting. Geochemistry, isotope geochemistry and geochronology study on the ca.2.5 Ga mafic dikes in the Zanhuang massif shows that they were generated from an enriched mantle source region, and may have formed in a subduction -related geodynamic setting. The ca.2.5 Ga mafic dikes are widely distributed in the Central Orogenic Belt and Eastern Block of the North China Craton, suggesting that they represent an important mafic dike swarm in the Neoarchean. The ca.2.5 Ga granitic rocks and mafic dike swarms in the whole North China Craton have a similar geodynamic setting. Accordingly, we suggest that there may be another important geological event on the western edge of the newly accreted orogenic belt after the ca.2.5 Ga arc-continent collisional event.5) We propose that the basement of the North China Craton experienced two stages of subduction/collision events during the period of Neoarchean (ca.2.5 Ga):(1) arc-continent collision:the Eastern Block of the North China Craton subducted to the northwest and collided with the Fuping arc terrane in the Central Orogenic Belt, resulting in formation of the Zanhuang melange; (2) subduction polarity reversed beneath the western margin of the arc-continent collision zone. Because of higher mantle temperatures and more buoyant oceanic lithosphere in the Archean, the ca.2.5 Ga arc-continent collision in the NCC may have blocked the continued subduction of the west-dipping subducting oceanic slab, and the convergence strength would make the western margin of the new accreted arc-continent to be a weak zone, then cause a gradual tearing and finally induce the reversion of subduction polarity. The enriched mantle starts to melt after the subduction reversal event and resulted in the development of the mafic dike swarms. The rising mafic magma that is parentally from melting of the enriched mantle ponded at the bottom of the old and thickened lower crust with arc affinity and gave rise to its melting and then produced the ca.2.5 Ga granitic rocks in the Central Orogenic Belt and Eastern Block of the North China Craton.6) This study provides remarkable evidence that the plate tectonics on the Earth started at Neoarchean. One of the unsolved key problems in earth sciences is when and how did the plate tectonics operate on the Earth, which is also one of the most intense debates in the Precambrain research worldwide. The subduction process and formation of suture zones are the basis of the plate tectonics. However, study on these two aspects has lagged far behind. Recognition of an ancient suture zone is one of the key marks for collision of different plates, and its identification can provide important constraints on the starting time for plate tectonics on the Earth. The Zanhuang melange is located between two blocks of the North China Craton-the Eastern Block and an accreted arc terrane (Fuping arc) in the Central Orogenic Belt. Detailed field and geochronology study show that the Zanhuang melange formed prior to 2.5 Ga. The clear association of rocks types and structures in the Archean Zanhuang tectonic melange suggest that modern-style plate tectonics was operating by the end of the Neoarchean.