Abstract

Controls on the formation and distribution of mineralization in continental collisional settings remain unclear. However, our synthesis of diverse geophysical data sets from the eastern margin of Tibet revealed that differential crustal rotation played a key role in the production of a variety of mineralization types. Due to Cenozoic continental collision between India and Eurasia, the elongated continental blocks in the eastern margin of Tibet were extruded and reoriented. Prior to block extrusion in the Eocene, two giant porphyry-skarn ore clusters formed at the boundaries between the central segment and both the northern and southern segments of the Jinshajiang-Ailaoshan suture zone. These crustal segment boundaries displayed counterclockwise rotation, due to clockwise rotation of the central segment relative to both the essentially immobile northern and southern segments, combined with crust-mantle decoupling. This is considered to have induced crustal friction and resultant generation of fertile magmas that formed the porphyry-skarn Cu-Au deposits. During Oligocene–Miocene block extrusion, differential rotation of upper crust occurred on the western and eastern sides of the north-northwest–trending Central Axis fault in the Lanping-Simao basin. Two Oligocene–Miocene Mississippi Valley–type ore clusters occur on fault segments with anomalous differential rotation of 70° to 80°, suggesting that this differential rotation resulted in local extension with consequent ore-fluid influx.

INTRODUCTION

Ore deposits normally form clusters that develop in specific geological time periods. An improved understanding of the control exerted by lithosphere-scale geological processes on this heterogeneous distribution should reveal the metal accumulation mechanisms that occur within these specific anomalous geological settings and, in turn, aid deposit targeting (Wilkinson, 2013). Previous research on this topic has mainly focused on the evolution of metal-bearing magmas and fluids in both active and passive plate boundaries (Lee et al., 2012). Comparatively, the controls on large-scale mineralization that relate to crustal deformation in continental collision settings are less understood.
Along the eastern margin of Tibet, clustered giant mineral deposits with diverse genetic types and metal speciation, including porphyry and skarn Cu-Au, and Mississippi Valley–type (MVT) Pb-Zn, formed during Cenozoic continental collision between India and Eurasia. The spatiotemporal heterogeneous distribution of Cenozoic crustal deformation has been identified from a combination of paleomagnetic studies and structural analyses of shear zones (Otofuji et al., 1990). By applying the paleomagnetic method, the main quantitative approach for understanding tectonic movements between crustal segments, and integrating seismic and gravity data, as well as the geochemical features of fertile intrusive rocks, our study revealed a genetic relationship between specific crustal deformation processes and the generation of giant ore clusters in the eastern margin of Tibet.

REGIONAL GEOLOGY AND METALLOGENY

Tectonic Setting

The eastern margin of Tibet is a collage of continental blocks that amalgamated due to the progressive Triassic to Cretaceous closures of the Paleo- and Neo-Tethyan Oceans. As a response to the closure of the Paleotethyan Ocean, the Triassic Lincang granite batholith intruded the western margin of the Indochina block. The Indian-Eurasian continental collision, which was initiated at ca. 55 Ma, caused both crustal thickening in Tibet and high-pressure granulite-facies metamorphism at ca. 40 Ma and ca. 37–32 Ma in the Eastern Himalayan syntaxis (EHS; Fig. 1; Ding et al., 2001; Zhang et al., 2010). The continuing impingement of the Indian continent on Eurasia caused the extrusion of elongated crustal blocks along the eastern margin of Tibet (Figs. 2A and 2B; Tapponnier et al., 1982). This extrusion reoriented the elongate crustal blocks from a northwest trend to a north-northwest trend (Fig. 2). During the collision, two Eocene porphyry-skarn ore clusters formed along the Jinshajiang-Ailaoshan suture zone (JAS), which extended along the margin of the East Qiangtang and South China blocks. Two MVT ore clusters also formed within the Lanping-Simao basin, in the northern Indochina block (Fig. 1).

Geology of the Jinshajiang-Ailaoshan Suture Zone

The JAS is largely composed of relics of the Jinshajiang-Ailaoshan Paleo-Tethys oceanic plate (Deng et al., 2014). In the Eocene, crustally derived potassic intrusive rocks, with emplacement ages of 42–34 Ma, and coeval extensional basins with intercalated potassic-ultrapotassic volcanic rocks formed mainly in the Xialaxiu to Yulong region and around the Jianchuan to Beiya region along the JAS (Fig. 1; Deng et al., 2014; Hou et al., 2017). Later lithosphere-scale sinistral ductile strike-slip shearing, mostly along the southern segment of the JAS, was initiated at ca. 31 Ma, culminated at ca. 27–21 Ma, and continued to ca. 17 Ma, contemporaneous with the reorientation of Indochina (Cao et al., 2011). In the JAS, the porphyry Cu ore cluster (>15 Mt Cu) formed near Yulong, and the porphyry-skarn Au ore cluster (>450 t Au) formed around Beiya (Hou et al., 2017; Fig. 1C; Fig. S1 and Table S1 in the Supplemental Material1).

Geology of the Lanping-Simao Basin

The Lanping-Simao basin, with over 2500 m of Mesozoic–Paleocene terrestrial siliciclastic sedimentary rocks containing intercalated salt horizons, was folded and faulted in the Cenozoic (Fig. 1; Deng et al., 2014). The north-northwest–trending Central Axis fault (CAF) developed in the center of the basin and transected the Mesozoic and Paleocene strata throughout the basin. Two Oligocene–Miocene MVT Pb-Zn polymetallic ore clusters, Jinding in the north (Zn + Pb reserves of >15?Mt) and Jinggu in the south, were distributed along with the CAF, with a few minor deposits formed along the basin boundary (Fig. 1C; Table S1). Available isotopic and paleomagnetic data suggest that the Jinding ore cluster formed in the transition from the Oligocene to Miocene (Leach et al., 2017). It has been suggested that evaporites in the Paleocene sedimentary sequence ascended gravitationally along a thrust fault to create the Jinding dome, which became a hydrocarbon and reduced-sulfur chemical trap that controlled deposition of subsequent Zn-Pb ores (Leach et al., 2017).

DIFFERENTIAL CRUSTAL ROTATION CONTROL ON MINERALIZATION

Differential Crustal Rotation and its Correlation to Eocene Mineralization
Tectonic rotation, as indicated by paleomagnetic declination data obtained from Cretaceous and Cenozoic sedimentary rocks, defines unequivocal regional differences between the various segments of the JAS (Fig. 1B). The northern segment from Erdaogou to Fenghuoshan consistently experienced no significant tectonic rotation (Fig. 1B; Table S2). However, the central segment between Gongjue and Jianchuan had a consistent clockwise rotation of ～20°. At the boundary between the two segments, however, paleomagnetic data near the Yulong ore cluster show that the JAS experienced significant counterclockwise rotation (Fig. 1C). The differential rotation sense indicates that this boundary zone represents the point of mechanical separation between the northern and central segments. This counterclockwise rotation on the western side of the JAS was most likely caused by eastward drag produced by the clockwise rotation of the central segment and the resultant differential rotation between the central and northern segments.
In contrast, the southern boundary of the central segment of the JAS, near the Beiya porphyry-skarn Cu-Au ore cluster, experienced counterclockwise rotation from ?24.7° ± 3.6° to ?11.8° ± 5.3° (Fig. 1B). However, as in the northern segment, the southern segment of the JAS from Jingdong to Yen Chau displayed insignificant rotation. Therefore, the counterclockwise rotation of the segment, including the Beiya ore cluster on the eastern side of the JAS, can also be explained by the westward movement of the central segment, which was detached from the southern segment.
Our study reveals a consistent crustal clockwise rotation of the central segment of the JAS during the Eocene. This clockwise rotation mechanically detached the central segment from both the northern and southern segments and induced the reversal in rotation of their boundaries where the Yulong and Beiya ore clusters are located (Fig. 2A).

Driver for Crustal Rotation and Generation of Fertile Magmatism

The angular deviation was calculated between the recorded orientation of SKS/SKKS shear wave splitting data (Chang et al., 2015), representing mantle lithosphere deformation, and the predicted fast-axis orientation calculated from the surface deformation field in the northern and central segments of the JAS (Fig. 1B; Table S3; Sol et al., 2007; Chang et al., 2015). The results revealed that the lithosphere in the two segments displays homogeneous deformation vertically, although the segment boundary containing the Yulong ore cluster displays decoupled crust-mantle deformation, as reflected by the anomalous average angular deviation (～20°; Fig. 2C). From the central to southern segment, the orientation of the SKS/SKKS shear wave splitting data transitions from a north-northwest to an east-west trend (Sol et al., 2007), most likely due to modification related to later mantle convection, considering that Neogene to Quaternary mafic rocks formed along the southern segment (Deng et al., 2014).
Paleomagnetic data collected from near Gongjue in the central segment were obtained from 53 to 43 Ma strata (Tong et al., 2017; Zhang et al., 2018), and hence the clockwise rotation should have occurred no earlier than ca. 43 Ma (Table S2). Previous studies have reported ca. 40 Ma (Ding et al., 2001) and ca. 37–32 Ma (Zhang et al., 2010) metamorphic events in the EHS, which are highly similar to the ages of the causative magmas and/or the mineralization itself in the Yulong (42–34 Ma) and Beiya (37–32 Ma) ore clusters (Deng et al., 2014). The Eocene crust in the segments hosting the Yulong and Beiya ore clusters was thickened by greater than 45 km, based on the high La/Yb and Sr/Y ratios of the causative potassic intrusions and the mineral compositions of enclosed lower-crustal xenoliths (Fig. S2; Table S4; Lu et al., 2013; Hou et al., 2017). Processing of the regional gravity data from the eastern margin of Tibet indicates a decline in the depth of the Moho interface from ～65 to 45 km along the northern and central segments of the JAS (Fig. S1). The crustal thickness estimated from gravity data is essentially equivalent to that obtained from Eocene igneous rocks (Hou et al., 2017), indicating that the crustal thickness structure in the eastern margin of Tibet has not significantly changed since the Eocene. Therefore, the spatial and temporal correspondence between individually determined counterclockwise crustal rotation, crust-mantle decoupling, and magma generation at segment boundaries advocates a genetic relationship among them.
We propose that indentation by the Indian continent induced the differential crustal rotation between the central and both northern and southern segments of the JAS, resulting in counterclockwise crustal rotation and crust-mantle decoupling on the segment boundaries that host the Yulong and Beiya ore clusters (Figs. 2A, 2C, and 3A). The decoupling is suggested to have caused frictional heating and consequent partial melting of the lower crust (Fig. 3A), in a manner similar to the shearing-induced melting of mafic rocks recorded in physical experiments (Hirose and Shimamoto, 2005). This partial melting of thickened lower crust would increase Fe3+ due to the preferential incorporation of Fe2+ into residual garnet (Tang et al., 2018), inducing more oxidized conditions in melts, which are critical for the behavior of metal species in differentiating magmas and the consequent formation of porphyry-skarn Cu-Au deposits. This new model differs from the delamination model of lower continental mantle lithosphere previously suggested to explain the potassic magmatism along the JAS (Lu et al., 2013).

DIFFERENTIAL ROTATION CONTROL ON MINERALIZATION IN THE LANPING-SIMAO BASIN

Differential Rotation and its Correlation to MVT Mineralization

Paleomagnetic data from the Lanping-Simao basin have illustrated that differential crustal rotation occurred on both sides of the CAF during Oligocene–Miocene block extrusion (Figs. 2B and 2D; Yang et al., 2001; Gao et al., 2015). The eastern part of the basin, from Weishan through Jingdong to Jiangcheng, has experienced only minor rotation (Fig. 1B). Consistently, relatively minor rotation of ～20° is shown for the Jianchuan basin, on the eastern side of the Lanping basin. By contrast, the western part of the basin records significant crustal rotation with an average difference of ～40° compared to the eastern part. Significantly, crustal rotation data from the part of the basin near the Jinding and Jinggu MVT ore clusters record the greatest difference of ～70° to 80°.
Paleomagnetic data from the part of the basin near Jinggu show it had experienced clockwise rotation of ～70° since ca. 35 Ma and ～10° after ca. 20 Ma (Fig. 1B; Gao et al., 2015). Therefore, rotation of ～60° occurred from ca. 35 Ma to 20 Ma, with similar large-scale rotations of the western part of the basin, particularly near the Jinding ore cluster, with a clockwise rotation of ～80° (Fig. 1B). This denotes a clear spatial correspondence between MVT mineralization (Yalikun et al., 2017) and large-scale differential crustal rotation in the Oligocene–Miocene.

Driver for Oligocene–Miocene Crustal Rotation and MVT Mineralization

Previous paleomagnetic data and structural analyses have shown that the rigidity of the Mesozoic Lincang granite batholith controlled tectonic rotation in the western part of the Lanping-Simao basin (Fig. 1B; Kondo et al., 2012). Fission-track dating of apatite from the Lincang batholith proved that rapid uplift and thrusting occurred at ca. 36–27 Ma in the southern segment of the batholith but was delayed in its central and northern segments until ca. 27–16 Ma (Shi et al., 2006). Thus, major deformation of the part of the basin hosting the Jinggu ore cluster should have occurred between ca. 27 Ma and ca. 20 Ma. Considering that this deformation mimics the rotation pattern of the western part of the Lanping-Simao basin, this suggests that the differential rotation between eastern and western parts of the basin, with consequent movement on the CAF, also occurred from ca. 27 to 20 Ma. This age span is consistent with the timing of formation of the giant Oligocene–Miocene Jinding ore cluster, confirming the synchroneity of differential crustal rotation and MVT mineralization in the basin. This rotation can be ascribed to northeast-trending compression induced by the northeastward movement of the rigid Lincang batholith as a consequence of the northeastward movement of the Indian continent (Figs. 2B and 2D).
The differential rotation between the eastern and western sides of the CAF would have facilitated the decoupling of basin strata and induced crustal rupture and resultant generation of a local extensional setting in the upper crust (Fig. 3B). This would have been a trigger for salt diapirism and subsequent ore-fluid accumulation in salt-related traps to form the MVT ore clusters along the CAF.

CONCLUSIONS

This study revealed that differential crustal rotation along an earlier suture zone, and within a sedimentary basin, along the eastern margin of Tibet occurred prior to and coeval with Eocene–Miocene crustal block extrusion, respectively. The differential crustal rotation was crucial in controlling the formation of giant ore clusters of porphyry-skarn Cu-Au deposits and MVT Zn-Pb deposits in the eastern margin of Tibet. This has implications for both the genesis and exploration of specific ore deposits produced in continental collisional settings.

ACKNOWLEDGMENTS

We thank editor Gerald Dickens and three anonymous reviewers for their insightful comments, and David Groves for English editing. This paper was funded by the National Natural Science Foundation of China (grant 918552171), the National Key Research and Development Project of China (grant 2016YFC0600307), the National Basic Research Program (grant 2015CB452600), and the 111 Project (grant BP0719021).
1Supplemental Material. Detailed description of data analyses, geological settings, Figures S1 and S2, and Tables S1–S4. Please visit https://doi.org/10.1130/GEOL.S.13232387 to access the supplemental material, and contact editing@geosociety.org with any questions.

? 2020 The Authors

https://doi.org/10.1130/G47855.1