2. Beijing Institute of Exploration Engineering, China Geological Survey, Beijing 100083, China;
3. Inner Mongolia Mining Development Limited Liability Company, Hohhot 010010, China
The magnetic fabric analysis can identify the state and direction of strain by analyzing the anisotropy of magnetic susceptibility (AMS) of rocks (Borradaile et al., 2012; Boummane and Olivier, 2007). Previous studies showed a relevant relationship of the shape and spatial orientations between the magnetic susceptibility ellipsoid and strain ellipsoid of rocks. Therefore, the AMS analysis methods are used to reveal the predominant orientation of the structural deformation and the emplacement of magmatic rocks (Borradaile et al., 2012; Rochette et al., 1999; Hrouda, 1982). Especially in the areas where it is complicated to unveil the tectonic evolution history, so magnetic fabric methods, coupled with the research methods of structural deformation, can be applied in tectonic studies to give more insights.
The southwestern Fujian rift basin (SFRB) is an Fe-Pb-Zn polymetallic and tectono-magmatic belt located at the southwestern margin of South China Block (Fig. 1), and it is also a typical geotectonic transition zone from EW-trending Paleotethyan tectonic domain to NE-trending circum-Pacific tectonic domain during late Middle Triassic to the Cretaceous, accompanied by multi-phase alternation of extensional and compressional tectonics (Wang et al., 2016, 2015a; Zhang et al., 2006). The Yanshanian movement occurred in the study area due to a large-scale movement of East Asian lithospheric plate, resulting in massive NE-and NNE-trending regional folds and thrust-nappe structures in the study area (Wang et al., 2015b; Zhang Z J et al., 2015; Zhang D et al., 2006). Due to the intensive tectono-magmatic activities during Late Mesozoic, massive geological structures overprinted each other and igneous rocks intruded or extruded in multiple periods in the SFRB.
The southwestern Fujian is well-known for the occurrence of large-scale iron polymetallic and gold ore deposits, such as the famous Makeng iron and Zijinshan copper (gold) deposits (Wang et al., 2015b; Zhong et al., 2014). Many studies have been carried out most of which only focused on the Mesozoic igneous rocks and the associated ore deposits in the southwestern Fujian metallogenic belt (Wang S et al., 2017a, b, 2015a; Li and Jiang, 2016; Zhao X L et al., 2015; Xu et al., 2013; Wang L J et al., 2007; Mao et al., 2006; Wang and Li, 2003). Thus understandings between the metallogenesis and other geological aspects of this belt are neglected, especially the tectonic evolution in this area. Therefore, understandings of tectonic settings and geodynamics interpretation of the SFMB and South China Block are inadequate and limited. Although previous research (e.g., Wang et al., 2015a; Mao et al., 2006) indicated that there was a multi-stage lithospheric stretching existed in southwestern Fujian during the Mesozoic, our understandings about the Mesozoic tectonic evolution have been hampered by the paucity of precise chronologic studies of the tectonic events. As a result, the controversy on the Late Mesozoic structural deformation and tectonic evolution of southwestern Fujian and the South China Block remains unresolved. It is anticipated that a precise geochronological and tectonic environment study of Mesozoic rocks, coupled with the magnetic fabric and structural deformation analysis, will improve our understandings of the Mesozoic tectonic evolution. Furthermore, basic dyke swarms are usually thought to be related to the extensional setting in southwestern Fujian (Ding et al., 2017; Wang S et al., 2016, 2015a; Wu et al., 2014; Wang K X et al., 2013; Peng et al., 2008; Hou et al., 2006a), so a geochronological study on these dykes will provide more information on the Mesozoic tectonic evolution in the study area.
According to the understandings above, the Makeng ore field in Longyan (Fig. 1) was selected as the study area to conduct the magmatic fabric and zircon U-Pb geochronological research. Together with previous studies, comprehensive analyses of geological structure, magnetic fabric, magmatic petrology and geochronology were done to provide more constrains on the tectonic evolution of SFRB during the Late Mesozoic.1 GEOLOGICAL SETTING
Located on the southeastern margin of the South China Block near the suture of the Yangtze and the Cathayia blocks, southwestern Fujian rift basin (SFRB) is an important part of the Mesozoic tectonic-magmatic zone on the west side of the Pacific Plate. The SFMB is a Late Paleozoic rift basin bounded by the NE-trending Zhenghe-Dabu fault on the west and EW-trending Ninghua-Nanping fault on the south (Fig. 1). This area experienced the Proterozoic South China Block forming and breaking, Late Paleozoic basin forming, and the Mesozoic tectono-magmatic events, and it is characterized by the alternation of compressional and extensional tectonics coupled with multiple phases of magmatism (Wang et al., 2015b; Zhang et al., 2004; Wang, 2003; Li, 2000). The strata are continuous in the SFRB on the whole, but the Silurian and the foot part of Devonian are absent. There are mainly three periods of rocks deposited in this area, including Pre-Devonian basement rocks, Carboniferous– Middle Triassic clastic rocks of sedimentary cover, and Jurassic–Cretaceous continental clastic and volcanic rocks (Mao et al., 2001). Among these strata, the Upper Devonian to Upper Mesozoic strata are the main sedimentary rocks that exposed in the study area, consisting of, from bottom to top, coarse detrital rocks of Taozikeng-Lindi (D3t-C1l), paralic calcareous rocks of Jingshe Formation-Qixia (C2j-P2q), and paralic to neritic fine detrital rocks of Wenbishan-Xikou (P2w-T1x).
Regional brittle faults and NE-trending composite folds were dominant among the geological structures in the study area. NE-and NW-trending faults widely developed in this area, cutting the study area into different rhombic blocks. The Xuanhe synclinorium, Damaoshan anticlinoria, Hufang-Yongding anticlinoria and Datian-Longyan synclinorium are the main folds in the study area. In addition, Mesozoic nappe structures widely developed, with thrusting direction from northwest to southeast, resulting in the stratigraphic deletion and duplication.
Vigorous magmatic activities occurred in several periods in South China Block, forming various igneous rocks. Among these rocks, Mesozoic acidic and intermediate-acidic magmatic rocks are dominant and widespread, especially the Late Mesozoic granites (130–150 Ma) which are related to the large-scale iron polymetallic deposits in the southwestern Fujian (Wang et al., 2015b). Additionally, many basic dykes are exposed in the SFRB in the form of diabase dyke swarms. These dykes are usually regarded as signs of extensional tectonics, indicating that southwestern Fujian with its adjacent areas experienced large-scale extension during Mesozoic (Wang et al., 2017a, 2015a).2 SAMPLING AND ANALYSIS 2.1 Magnetic Fabric Analysis
In this research we chose the following rocks for the magnetic fabric analysis: coarse grained detrital rocks of the Lower Carboniferous Lindi Formation (C1l), limestones of Upper Carboniferous Jingshe-Upper Permian Qixia Formation (C2j-P2q), fine grained detrital rocks of Upper Permian Wenbishan-Tongziyan Formation (P2w-P2t), Juzhou granite and diabase dykes. A total of 311 samples were collected in-situ from 42 geological sites in the study area (Fig. 1). These samples were gathered systematically, using a portable gasoline-powered drill oriented by a magnetic compass.
The recovered cores with a diameter of 2.5 cm were cut into cylinders with 2.2-cm-lengths in the laboratory, and then selected for magnetic fabric measurement at the Paleomagnetism and Environmental Magnetism Laboratory of China University of Geosciences (Beijing). The MFK1-FA model Kappa Bridge magnetic susceptibility instrument was used under the following conditions: magnetic field intensity of 300 A/m, operating frequency of 875 Hz, environment temperature of 20 ℃, an sensitivity of 2×10-8 SI, and accuracy of 0.1%. Fifteen magnetic susceptibility values of the three principal axes (K1, K2, and K3) were obtained using the least square method, and the measurements was done by rotating the sample in different directions. The obtained data were synthesized using the Anisoft 4.2 software to get the deformation parameters such as Km (average magnetic susceptibility), P (anisotropy of magnetic susceptibility), E (ellipsoid flat rate of magnetic susceptibility), T (ellipsoid shape factor of magnetic susceptibility), F (magnetic foliation), L (magnetic lineation), etc. (Table 1).
Representative samples of Juzhou granites and diabase dykes were collected from Makeng area for zircon U-Pb dating. Zircon grains were picked out from the samples by employing a combination of standard heavy liquid and magnetic separation techniques at the Institute of Regional Geology and Mineral Resources Survey in Hebei Province. These zircons were mounted onto an epoxy resin disk, polished to approximately half of the zircon model grain thickness. Prior to U-Pb isotopic analysis, the internal structures of the zircons were imaged by cathodoluminescence (CL) techniques at the SHRIMP Centre of Chinese Academy of Geological Sciences in Beijing. Analyses of the zircons were conducted using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the Geological Lab Center of Tianjin Institute of Geology and Mineral Resources. Full details on the LA-ICP-MS analytical procedures are described in Deng et al. (2013). A standard sample was measured after every eight measuring points to monitor the stability of the instrument and the accuracy of the ion counting statistics. After the analyses, GLITTER 4.4 software was applied to calculate the 207Pb/206Pb, 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios, and common Pb was corrected using the method from Andersen (2002). Weighted mean U-Pb ages and concordia plots were calculated using ISOPLOT 3.0, with a degree of uncertainties quoted at 1σ and 95% level of confidence (Ludwig, 2003).3 MAGNETIC FABRIC RESULTS 3.1 Sedimentary Rocks
Due to unfavorable conditions of the vegetation coverage, intense erosion, weathering and magmatic reworking, it is difficult to carry out researches on the Late Mesozoic structural deformation in southwestern Fujian. Furthermore, the absence of structural marker beds hindered the determining of strain ellipsoid using conventional macroscopic structural analysis. Considering these factors mentioned above, a magnetic fabric study on the main types of sedimentary rocks in Makeng area was carried out to provide more constraints on the structural deformation in the SFRB.
Samples from Lindi (C1l), Jingshe-Qixia (C2j-P2q), and Wenbishan-Tongziyan (P2w-P2t) formations were collected from 33 geological sites. Magnetic fabric results showed conspicuous variations of different average values of magnetic susceptibility (Km) among these lithologies, with most values varying from 101 to 61 700 μSI shown in Table 1. On the whole, the values of anisotropy magnetic susceptibility (AMS) are relatively low, with adjusted average values of AMS (Pj) ranging from 1.011 to 1.907 (Pj > 1.05, mostly). There is no significant correlation between the Km and Pj values, indicating that the AMS is caused by post tectonic activities (Deng et al., 2013). Overall, The F (magnetic foliation) values are significantly greater than the L (magnetic lineation) values (Fig. 2 and the T values representing the AMS ellipsoid flat rate is generally greater than 0 (Figs. 2 and 3). In addition, a portion of the magnetic parameters are characterized by F < L and T < 0 in the study area, which are usually regarded to be associated with extensional tectonics (Kissel et al., 2010). According to features of the magnetic foliation, magnetic lineation and the AMS principal axis orientations, the structural deformations of these rocks are classified into four groups: NW-trending compressional, NE-trending compres-sional, NW-trending extensional and NEE-trending extensional structures.3.1.1 NW-trending compressional structures
Most of the samples collected from D01, D02, D05, D11, D18, D26, D29 and D31 sites showed characteristics of F > L, E > 1 and T > 0, indicating the development of magnetic foliation. These samples fell into the oblate deformation area (E > 1) in the magnetic fabric Flinn diagrams (Fig. 2) and T-Pj (Fig. 3) graphs. The dominant orientation of the minimum principal axis of magnetic susceptibility (K3) is conspicuous, with pole density center distributing in the second and the fourth quadrants and a low dip angle varying from 1° to 31° (Fig. 4).3.1.2 NE-trending compressional structures
The majority samples from D03, D06, D07, D10, D19, D20, D21, D22, D23, D25, D28 and D30 sites are characterized by E > 1, T > 0 and F > L, falling into the area below the line of F=L (Figs. 2, 3). Most samples showed conspicuous NNE-SSW oriented minimum principal axes of magnetic susceptibility (K3), with the pole density center distributing within the first and third quadrants (Fig. 4). Magnetic fabric features of these rocks indicate an oblate strain ellipsoid, and advocate for an NE-SW trending compressional strain of these rocks.3.1.3 ENE-trending extensional structures
Most samples from D12, D15, D17 and D27 sites are characterized by L > F, E > 1 and T < 0, falling above the line of F=L and within in the prolate area in the Flinn (Fig. 2) and T-Pj diagrams (Fig. 3), respectively. The maximum principal axes of magnetic susceptibility (K1) of these samples are dominantly in the direction of ENE (Fig. 4). The magmatic fabric characteristics above are characterized by a prolate strain ellipsoid, indicating an ENE-WSW trending extensional deformation.3.1.4 NW-trending extensional structures
Samples from D04, D08, D09, D13, D14, D24 and D33 sites are characterized by L > F, E > 1 and T < 0, with the maximum principal axis of magnetic susceptibility (K1) dominantly pointing to NW (Fig. 4). These features mentioned above indicated the prolate strain ellipsoids of magnetic susceptibility and advocated for NW-SE trending extensional strain.3.2 Juzhou Granite
Located on the southeastern side of Makeng iron ore deposit, Juzhou granite is closely associated with the metallogenesis of this deposit (Wang et al., 2015b). To reveal the emplacement mechanism of Juzhou granite, 7 geological sites (from D34 to D40) were selected for the magnetic fabric analysis.
Average values of magnetic susceptibility (Km) of most of the samples are varying between 147 to 717 μSI (Table 1), lower than 500 μSI on the whole, indicating that magnetic susceptibility of these samples are not caused by the ferromagnetic minerals such as magnetite (Rochette et al., 1999). The average Pj values of these samples varied from 1.022 to 1.140 (Pj < 1.2), which can be concluded that the magnetic fabrics of Juzhou granites are caused by primary magma flows (Hrouda, 1982). The susceptibility ellipsoid magnitude parameters of the Juzhou granite are characterized by F > L, E > 1 and T > 0. These samples plot within the oblate deformation area (E > 1) below the line of F=L in the magnetic fabric Flinn diagrams (Fig. 5), illustrating the more developed magnetic foliation flaser type strain ellipsoid. The pole density center of K3 falls into the first and the third quadrants with NNE-SSW dominant orientation (Fig. 5).3.3 Diabase Dykes
Late Mesozoic diabase dykes spread widely in southwestern Fujian, and they are regarded as an important discriminant of extensional tectonics in this area (Wang et al., 2017a, 2015a). These diabase dykes are strongly weathered and altered due to the hot and humid climate, so it is difficult to determine their precise attitudes. Therefore, it is necessary to carry out the magnetic fabric research on the dykes in order to shed light on its relation to the Mesozoic extensional structure.
Two representative NW-trending diabase dykes were selected in Makeng area to conduct magnetic fabric analysis. Among these samples (D41 and D42), the Pj values of AMS varied from 1.005 to 1.007, and the average magnetic susceptibility Km values varied from 544 to 627 μSI (Table 1). The variations proved that these rocks have suffered a weak or non-reworking by later tectonic activity. Generally, these diabase samples are characterized F > L, falling in the oblate deformation area (E > 1) in the Flinn diagrams (Fig. 5). The minimum principal axis of magnetic susceptibility (K3) is oriented in the ENE-WSW, nearly perpendicular to the wall rock, implying that its occurrence resulted from the magma flows (Pan et al., 2012).4 ZIRCON U-PB DATING RESULTS 4.1 Juzhou Granite
Zircon grains of Juzhou granite (Sample D3086) are euhedral, with clear oscillatory zonings on their edges (Fig. 6a), and their length varying between 100 and 200 μm. The CL images showed that these zircons are complete without serious radiation damages. The morphological characteristics of the zircon grains and the magmatic oscillatory zonings indicate that they are magmatic zircons (Belousova et al., 2002). This is further supported by the Th/U ratios which are mostly higher than 0.1 (Th/U=0.33–1.28) except #19 (Corfu et al., 2003). Most of the analyses (except #12, which was excluded) yielded 206Pb/238U apparent ages between 129±1 and 137±1 Ma, with a weighted mean 206Pb/238U age of 132.4±0.8 Ma (MSWD=3.5; Fig. 6b).4.2 Diabase Dykes
Most zircons collected from diabase dyke (Sample D3097) have euhedral and elongated shapes. The zircon crystals are mainly colorless, with their length ranging from 70 to 110 μm. The CL images showed that zircons are complete without suffering significant damages. Most of these zircon crystals have oscillatory zonings in the CL imaging (Fig. 6c). The oscillatory zonings indicate that these zircons are magmatic origin. The zircon U-Pb isotopic analysis results for Sample D3097 are presented in Table 2, and nine analyses were carried out on the rims of zircon these grains. Several zircons were scatter in the concordia diagram (Fig. 6c) due to the Pb loss caused by the fractures in the minerals, possibly. On the whole, the majority analyses points are concordant except #1 and #6 (captured zircons), these zircons are distributed on and near the concordant line with concentrated 206Pb/238U ages varying between 140±2 and 144±1 Ma, averaging at 141.33±0.96 Ma (MSWD=1.02; Fig. 6d). Based on the morphological characteristics and high Th/U ratios (Th/U > 0.1, Corfu et al., 2003) of these zircons, we considered that zircon U-Pb age of 141±1 Ma can represent the magmatic crystallization time of the diabase dykes.
The genesis of the Makeng iron deposit has been a controversial topic for decades, and recent researchers advocated that the formation of magnetite is cogenetic to the Juzhou granite to some extend (Wang et al., 2015b; Zhang and Zuo, 2014; Mao et al., 2006). Mao et al. (2006) obtained U-Pb ages of 136.0 and 133.8 Ma from single zircon crystals for Juzhou granite. Zhang et al. (2013) carried out LA-ICP-MS zircon U-Pb dating and obtained weighted mean 206Pb/238U ages of 125–130 Ma for Juzhou granite. Our zircon U-Pb dating obtained 206Pb/238U ages varying between 129±1 to 137±1 Ma with a weighted mean 206Pb/238U age of 132.4±0.8 Ma. These research results mentioned above indicated that Juzhou granite is a composite intrusion, with a crystallization time of more than 10 Ma possibly. In the dated 128 zircons (U-Pb system), 118 yielded 206Pb/238U ages ranging from 119 to 141 Ma for Juzhou granite, with the peak U-Pb ages ranging from 123 to 135 Ma (Fig. 7). On the whole, the emplacement age of the Juzhou granite suit may be generally limited to 120–140 Ma, in the early stage of Early Cretaceous.
The emplacement and formation of granites is a significant part of the earth system, and it is also the key to understand the tectonic evolution and geodynamics of orogenic belts. Magnetic fabrics of the magnetic minerals showed a preferred orientation during the emplacement and crystallization of the granite, which caused the anisotropy in the magnetic susceptibility (Boummane and Olivier, 2007). Magnetic fabrics of granites are relatively stable after crystallization, and it hardly changes in the case of brittle deformation. Therefore, it records the information of magma generation, propagation, crystallization and deformation. The magnetic fabric analysis method was successfully conducted by researchers to illustrate the emplacement mechanism of granites (Pan et al., 2012; Boummane and Olivier, 2007; Nagaraju et al., 2007; Stevenson et al., 2007).
The Juzhou granite depicts an anisotropic magnetic susceptibility, with magnetic fabrics inheriting from primary magma flows. There is no significant correlation between the Km and Pj values (Fig. 8), which indicates that the AMS variations were not caused by an uneven distribution of magnetic minerals, but the tectonic strain (Deng et al., 2013). There seems to be a preferred orientation of the K3 of the Juzhou granite, because the dense pole centers of the four groups of samples plot into the second and fourth quadrants in the AMS fabric diagram (Fig. 5). This portrays a NE-SW trending compressive strain ellipsoid. Thus generally, it can be said that the magnetic fabrics of the granite samples are characterized by magnetic foliation and flattening strain ellipsoid, which indicates that the Juzhou granite formed in the NE-SW trending compressional stress field during the late stage of Early Cretaceous.5.2 Emplacement Age and Tectonic Implications of Diabase Dykes
Originating from deep earth and intruding into different rock units, basic rocks are usually thought to be an adequate evidence for extensional tectonics (Wu C Z et al., 2014; Wang K X et al., 2013; Peng et al., 2008; Hou et al., 2006b). The widespread diabase dykes in southwestern Fujian and its neighboring areas attest that it experienced extensional tectonics during the Late Cretaceous. The LA-ICP-MS zircons U-Pb dating of the diabase yielded 206Pb/238U ages of 140 to 144 Ma with a weighted mean age of 141±1 Ma, corresponding to the early stage of Early Cretaceous. The analyzed zircon grains are of magmatic origin, thus the age of 141±1 Ma represent the time these diabase dykes emplaced.
Usually, the dominant orientation of the minimum principal axis of magnetic susceptibility (K3) is nearly perpendicular to the diabase wall rocks, and the magnetic foliation is nearly parallel to the wall rock (Kissel et al., 2010; Rochette et al., 1999). Therefore, studying the magnetic fabrics of these dykes can determine the directions in which the magma emplaced. Magnetic fabric strain ellipsoids of these diabase samples are oblate type with a mean orientation of minimum principal axis of magnetic susceptibility (K3) pointing to ENE-WSW, this indicates that these diabase dykes are NNW-SSE trending. Thus, the trend direction of these dikes (ENE-WSW) and their strain ellipsoid (oblate) features revealed the existence of ENE-WSW trending extensional structures in Early Cretaceous in southwestern Fujian.5.3 Late Mesozoic Tectonic Evolution
Together with former studies, magnetic fabric, geochronology and structural deformation research in this paper revealed that it mainly experienced five tectonic evolution stages during Late Mesozoic in southwestern Fujian and its adjacent areas: J1-J21 extension (200–170 Ma), J22-J3 nappe structures (170–145 Ma), K11 extension (145–135 Ma), K12 compression (135–123 Ma) and K21 extension (123–85 Ma).5.3.1 J1-J21 extension (ca. 200–170 Ma)
Vast previous studies showed that extensional tectonics existed in the southwestern Fujian and its adjacent areas during Early Jurassic to the early stage of Middle Jurassic (Zhou et al., 2006; Xing et al., 2002; Xu, 1992). Early Jurassic bimodal volcanic rocks in the Yongding Basin yielded Rb-Sr isochron age of 179 Ma, and their Sr isotopic features suggest they were of mixed crust-mantle magma provenance, indicating these rocks formed in an intraplate rift setting (Xu, 1992). Another report discussed about the Early Jurassic bimodal volcanic rocks associated with the extension in southwestern Fujian, with K-Ar diagenetic ages of 177–188 Ma, and it is also thought to be a mark of the end of Indosinian intracontinental orogeny (Xing et al., 2002). Early Jurassic basalts and gabbros are widely exposed in the Yongding area, with Re-Os age of 175.4±3.1 Ma for the Fankeng basalts, which is thought to derive from the asthenosphere due to the upwelling of asthenospheric mantle (Zhou et al., 2006). A systematic study of the Tangquan granite in southwestern Fujian by Mao et al. (2004), obtained the petrogenetic ages of 183–158, and 182–162 Ma, and the later rocks are thought to form in a slow cooling (4.76 ℃/Ma) phase, which indicates that they formed in an extensional setting in Early Jurassic to the early stage of Middle Jurassic. According to previous studies mentioned above, it can be inferred that southwestern Fujian experienced extensional tectonic from Early Jurassic to the early stage of Middle Jurassic.5.3.2 J22-J3 nappe structures (ca. 170–145 Ma)
The Middle to Late Jurassic is an important tectonic phase in the SE China Block (especially in the SFRB), and it is marked by the widespread nappe structures due to the regional NW-SE trending extrusion. Former studies confirmed that the NE-SW-trending nappe structures and their associated magmatic rocks existed widely in southwestern Fujian (Jiang et al., 2015; Zhang et al., 2006; Mao et al., 2001). Zircon U-Pb dating yielded weighted mean ages between ca. 165 and 157 Ma for the monzogranite from Zijinshan ore field in southwestern Fujian, and these monzogranitic plutons are thought to be related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent (Jiang et al., 2015). It yielded petrogenetic ages of 145–151 Ma for the monzogranites in Hugang area of southwestern Fujian, and the geochemical features of these monzogranites indicated that they formed in compressional tectonic environment. Recent studies showed that the widespread NE-SW-trending nappe structures in the SFRB, together with the NE-SW trending folds, thrust faults, and diabase dykes mainly formed in Middle to Late Jurassic with structural-petrogenetic ages ranging from 150 to 154 Ma (Lü et al., 2014; Wang S et al., 2013; Zhang et al., 2006).
In this paper, compressional structures of the NW-SE direction were discerned by magnetic fabrics of the samples collected from the Lindi (C1l) and Tongziyan (P2t) formations. The anisotropies of magnetic susceptibility of these rocks are characterized by the oblate type strain ellipsoids and magnetic foliations, with the dominant orientations of the minimum principal axes of magnetic susceptibility (K3) pointing to NW or SE.
In addition, the NW-SE-trending geological structures developed widely in the study area, which testified the implications of the magnetic fabrics and the previous understandings about the Middle to Late Jurassic compressional tectonic regime in southwestern Fujian (Wang et al., 2016; Zhang et al., 2013). More evidence for the regional compression of NW-SE direction was provided by the structural deformation research from the SFRB, for example, the nappe structures with kinematic directions from NW to SE, fold structures with attitudes of axial surfaces trending to NW and the conjugate shear joints with dominant orientation of the maximum principal stress (σ1) pointing NW or SE (Fig. 9).
On the whole, it can be inferred that the southwestern Fujian experienced an important compressional tectonic (mainly nappe structures) stage, with the kinematic direction of NW-SE, from the Middle to Late Jurassic (ca. 170–145 Ma).5.3.3 K11 extension (ca. 145–135 Ma)
A transition from compressional to extensional tectonic regime occurred in the SE China Block at the beginning of Cretaceous, and it is characterized by extensional structures and the associated basic magmatic activities in the SFRB during the early stage of Early Cretaceous (Wang et al., 2015a; Xing et al., 2008; Yu et al., 2006; Zhang et al., 2001). Early Cretaceous fault basins, intrusive magmatism and volcanism related to extensional tectonics developed in southern China, with the peak magmatic ages at 135±5 Ma (Shu et al., 2008; Zhou et al., 2006; Li, 2000). According to the reports of Yu et al. (2006), the basaltic extrusive rocks in the Ganzhou-Hangzhou tectonic belt of southeastern China yielded ages of 143±1.1 to 139±0.7 Ma, and the geochemistry indicated that these rocks formed in the stretching tectonic environment. The Early Cretaceous volcanic rocks of Nanyuan Formation from Xianyou Country yield an age of 142.3 Ma, with geochemical features indicating an extensional tectonic setting (Xing et al., 2008). The syntectonic granitoid dyke in Guangping was dated at 142±1 Ma, representing the beginning of an extension after the end time of the NW-SE trending nappe structures (Lü, 2014). The former research mentioned above indicate that southwestern Fujian and its adjacent areas experienced extensional tectonics in the early stage of Early Cretaceous at ca. 145–135 Ma.
The zircons grains picked from the NNW trending diabase dykes in Makeng mine field yielded 206Pb/238U ages varying between 140±2 and 144±1 Ma, with a weighted mean age of 141.33±0.96 Ma. Usually, diabase dykes are regarded as a typical discriminant for the extensional tectonic environment (Wang S et al., 2015b; Wu et al., 2014; Wang K X et al., 2013; Peng et al., 2008; Hou et al., 2006a). Therefore, widespread diabase dykes in the study area indicated that the stretching tectonics existed in southwestern Fujian at about 141 Ma, and the magnetic fabric orientations of diabase dykes constrained an ENE-WSW trending extension in this area.
From the field study, a series of NNW-trending extensional faults, detachment folds and diabase dykes were discovered in southwestern Fujian (Fig. 10). These observations further attested to the existence of ENE-WSW extensional tectonics in the SFRB during the early stage of Early Cretaceous.5.3.4 K12 compression (ca. 135–123 Ma)
Early Cretaceous compression was mainly reflected by the extruding structures and the emplacement mechanism of the contemporary granites. Magnetic fabrics of Juzhou granite are characterized by the magnetic foliation and prolate type strain ellipsoids, with the maximum principal compressive stress (K3) directions pointing to NNE-SSW. These magnetic fabric features indicated that the Juzhou granite formed in NE-SW trending compressional stress field.
Zircons U-Pb dating of Juzhou granite yielded 206Pb/238U ages varying from 129 to 137 Ma, and the dating result provided time constraints not only for the emplacement of the granite, but also for the regional NE-SW-trending compression. A survey of all the U-Pb dating data of the Juzhou granite yielded concentrated 206Pb/238U ages of 119–141 Ma, with peak values varying between 123 to 135 Ma, standing for the main time phase of its emplacement. In addition, dating results of the granites from Yangshan, Pantian and Luoyang areas yielded U-Pb ages of 130, 129–132, and 131.7 Ma, respectively, with geochemical features of arc volcanic or active continental margin geotectonic setting, indicating the petrogenesis of these rocks are related to the subduction tectonic event (Lü, 2014).
In addition, Magnetic fabrics of sedimentary rocks (from D03, D06, D07, D10, D19, D20, D21, D22, D23, D25, D28 and D30 sites) in the study area showed features of oblate strain ellipsoid, advocating for a NE-SW trending compressive stress in the study area. NE-SW trending compression is also reflected by structure deformations in the field, for example, the fold structures with axial surface attitudes of NE direction, the shear fractures caused by NE-SW compression, and the conjugate shear joints with maximum principal stress (σ1) orientation of NE-SW (Fig. 11). All in all, it can be concluded that the NE-SW trending compressional structures existed widely in the SFRB during the late stage of Early Cretaceous ca.135–123 Ma.5.3.5 K21 extension (ca. 123–85 Ma)
Following the NE trending compression, the study area began to experience a regional extension in the early stage of Late Cretaceous, and the extension in the period of ca. 125–185 Ma were advocated by many scholars (Xu et al., 2014; Xing et al., 2009; Dong et al., 2006; Mao et al., 2001). Geochemical and geochronology researches on Late Cretaceous intermediate-acidic rocks in southwestern Fujian indicated that they belonged to post-orogenic granite series and formed in an extensional lithospheric setting (Mao et al., 2001). Late Cretaceous high-angle normal faults, low-angle normal faults and detachment faults are developed widely in Quanzhou area, and they were deemed to form in the NE-SW extensional setting (Xu et al., 2014). A-type granites from the southeastern coast of Fujian Province yileded zircon U-Pb ages of 92–86 Ma, and the generation of these rocks was thought to be related to the intensive tectonic extension along the Changle-Nan'ao fault during the Late Cretaceous (Zhao J L et al., 2015). Zircon U-Pb dating of the volcanic rocks in Shanghang Basin restricted the volcanism in the period between ca. 105 and 99 Ma, these rocks formed in a lithospheric extension setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent (Jiang et al., 2015).
The NW-SE trending compression is well reflected by the magnetic fabric studies in the study area, and it was attested by the prolate type strain ellipsoids and the development of magnetic lineation, with the NW-SE orientations of the maximum principal axis of magnetic susceptibility (K1). Field geological surveys showed that, the detachment faults caused by the NW-SE extension were widespread in the SFRB, and they are mainly normal faults and bedding detachment structures (Fig. 12). Thus, it can be inferred that the NW-SE trending extensional tectonics in southwestern Fujian existed widely during the early stage of Late Cretaceous.6 CONCLUSION
(1) Based on the AMS analysis results of Late Paleozoic sedimentary rocks, we divided the structural deformations in southwestern Fujian into four main groups: NW-SE compression, NNE-SSW compression, ENE-WSW extension, and NW-SE extension.
(2) LA-ICP-MS zircon U-Pb analysis dating of Juzhou granite yielded 206Pb/238U ages varying between 129±1 to 137±1 Ma, with a weighted mean age of 132.4±0.8 Ma. The emplacement ages of Juzhou granite were constrained to about 135–123 Ma. The magnetic fabric features of Juzhou granite indicated that it formed in NE-SW trending compressional stress field in southwestern Fujian.
(3) Zircon U-Pb analysis for diabase dykes yielded 206Pb/238U ages of 140 to144 Ma with a weighted mean age of 141±1 Ma, and they were regarded as the product of extensional environment. Magnetic fabric features delineated the NNE-SSW spreading direction of these diabase dykes, indicating the existence of the ENE-WSW trending extension in southwestern Fujian during the early stage of Early Cretaceous.
(4) The Late Mesozoic tectonic evolutions in southwestern Fujian were divided into five main stages: Early Jurassic extension, Middle–Late Jurassic nappe structure, early stage of Early Cretaceous extension, late stage of Early Cretaceous compression and Late Cretaceous extension.
This work was supported by the projects the China Geological Survey (Nos. 12120113089600, 12120114028701 and 1212011085472), the Key Project of Natural Science Foundation of China (No. 41530321) and the Fundamental Research Funds for the Central University (No. 2652017259). We are grateful to Mr. Tao Guo from Institute of Geomechanics, Chinese Academy of Geological Sciences and Mr. Jikai Ding from China University of Geosciences for providing sampling equipment and data processing, respectively. Finally, we appreciated two anonymous reviewers and editors for their valuable comments on this article. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0968-5.
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