| Citation: |
Zhi-yu Yi, Si-lin Yang, Joseph G. Meert, Xu-xuan Ma, 2023. Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision, China Geology, 6, 269-284. doi: 10.31035/cg2022077
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Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision
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Abstract
This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa. Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision. Magnetite is identified as the main magnetic carrier in our study. The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations. A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks. The authors interpret this AMS fabric to have formed during intrusion rather than deformation. Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities. A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation. The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N±5.8°N for the southern margin of Asia near Lhasa. The paleolatitude corresponds to an in-between position of the Lhasa terrane during about 130‒60 Ma. Furthermore, the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18°±9° for the sampling location since about 83 Ma. In the light of tectonic setting of the dykes, the strike of the southern margin of Asia near Lhasa is restored to trend approximately about 310°, which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.
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References
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Zhi-yu Yi, Si-lin Yang, Joseph G. Meert, Xu-xuan Ma, 2023. Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision, China Geology, 6, 269-284. doi: 10.31035/cg2022077
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Figure 1.
a–Tectonic framework of the Lhasa Terrane and adjacent area (after Enkin RJ et al., 1991); b–simplified regional geologic map of the Gangdese magmatic arc near Lhasa showing distribution of the Mesozoic and Cenozoic rock units and paleomagnetic sampling locations.
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Figure 2.
Field photos showing distribution of the sampling sites (b) and contacts between the diorite dykes and the country rock (a); plot of the bedding attitutes of sampled diorite dykes (c).
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Figure 3.
Optical petrography (a, c) and scanning electron microscope (SEM) observations (b, d) on the thin sections from the diorite dykes (c, d) and granodiorite country rock (a, b) indicate the magnetic minerals are magnetite or titanomagnetite which have not experienced visible alteration. Mineral orientation with insignificant deformation can be observed in (d). Abbreviations: Mag‒Magnetite. See text for more information.
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Figure 4.
Stereoplots of three principal axes of anisotropy of magnetic susceptibility (AMS) ellipsoid in geographic (a) and paleogeographic (b) systems and plots of AMS degree (Pj) versus total susceptibility (c) and AMS shape (T) (d) respectively. The pilot samples marked with red color are from granodiorite country rocks and the others are from dykes; the grey dashed arc in (a) represents the mean bedding plane of the dykes.
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Figure 5.
Representative results of hysteresis loops of the pilot samples from the diorite dykes (a‒g) and granodiorite country rock (h) with maximum field of 0.4 to 1.0 Tesla. Acquisition of isothermal remanent magnetization (IRM), back-field demagnetization curves (in the second quadrant of each diagram) with maximum fields of 0.5 T. (i) Day plot (Day R et al., 1977; Dunlop DJ, 2002) of hysteretic parameters calculated from (a‒h) showing magnetic particles are plotted within the pseudo-single domain (PSD) range; (j) Representative FORC measurement showing magnetic particles are dominant by larger PSD or MD particles, see text for more explanation.
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Figure 6.
Orthogonal (Zijderveld) vector plots of representative specimens from the dykes (a‒e, g‒i) and the country rock (f). Directions are plotted in-situ; open and solid circles represent vectors and endpoints projected onto horizontal and vertical planes, respectively.
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Figure 7.
Equal-area projections of site-mean directions of the ChRMs before and after tilt-correction for the diorite dykes and granodiorite country rocks from the Gangdese Belt, southern Tibet. Solid/open symbols represent downward/upward inclinations; stars indicate the overall mean directions with 95% confidence limits.
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Figure 8.
Virtual geomagnetic poles (VGPs) of site-mean directions (stratigraphic coordinate) on equal-area projections. The VGP cutting-off is based on Vandamme D (1994). ASD–angular standard deviation; N–number of the VGPs; Dec–magnetic declination; Inc–magnetic declination; a95–95% confidence interval of the mean paleomagnetic pole; SB–the dispersion of VGPs and its lower (Sl) and upper (Su) limits.
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Figure 9.
Comparison of paleolatitudes of India, Eurasia, Himalayas, and Lhasa since late Cretaceous. The numbers 1, 2, 3 represent paleolatitudes for the Lhasa terrane obtained from the Gangdese pluton (dykes and country rock), Dianzhong (Yi ZY et al., 2021 compiled from Chen JS et al., 2010; 2014 and Huang WT et al., 2015) and Nianbo (Chen JS et al., 2010, 2014; Liebke U et al., 2010; Sun ZM et al., 2010) formations, whilst the numbers 4, 5, 6 indicate paleolatitudes for the Himalayas otained from the Zongshan (Yi ZY et al., 2016 revised from Patzelt A et al., 1996), lower Zongpu and upper Zongpu (Yi ZY et al., 2011) formations; Number 7 and 8 represent paleolatitude resolved from the Cailangba (Jiangzi) and Mubala (Saga) sections (Yuan J et al., 2021). See text for more explanations.
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Figure 10.
(a) Reconstructions of the southern margin of Eurasia prior to the India-Asia collison (about 85 Ma). Precollisional positions of the southern margin of Asia in its western and eastern segments are constrained by paleomagnetic data from Yi ZY et al. (2015) and this study, respectively. The precollisional strike of the southern margin of Asia at about 80°E (b) is restored on paleomagnetic data obtained from the late Cretaceous volcanic rocks in the Shiquanhe and Yare basins according to Yi ZY et al. (2015) ; the precollisional strike of the southern margin of Asia at about 90°E (c) is inferred from the original strike of the late Cretaceous dykes (this study) in consideraion that the dykes were generated by an arc-paralelled extension caused by Neo-Tethys subduction in the late Cretaceous (Ma XX et al., 2017). solid lines and arrows in green in (b) and (c) were restored by aligning the documented declinations (solid lines and arrows in green) to the geographic north; Eurasia, Africa, Arabia and India in (a) are positioned in accordance with the APW path of Torsvik TH et al. (2012); the Neo-Tethyan subducted slab is constructed according to Van der Voo et al. (1999). Abbreviations: IN‒India; GI‒“Greater India”; AB‒Arabia; AF‒Afrian. The “Greater India” of about 1500 km is defined according to Yi ZY et al. (2011).