Geochemical Characteristics and Tectonic Significance of Shibandun Mafic-ultramafic Cumulates in Western Part of the Central Qilian

doi:10.19657/j.geoscience.1000-8527.2018.04.04

Abstract

Abstract:

The Shibandun mafic-ultramafic cumulates are widely distributed in the western part of the Central Qilian, north of the Danghenanshan fault zone. The mafic-ultramafic cumulates comprise multiple sequences of (from bottom to top) peridotite, olivine pyroxenite, gabbro, magnetite and diorite. LA-ICP-MS zircon U-Pb dating of the gabbro yielded a mean ²⁰⁶Pb/²³⁸U age of (486.5±3.3) Ma. Geochemical analysis indicates that serpentinized olivine pyroxenite has similar multi-element patterns to the gabbros, and they both show large ion lithophile element (LILE) enrichments and high field strength element (HFSE) depletions. They are also relatively enriched in LREE, and with flat HREE patterns and positive Eu anomalies (0.97-3.64). The study shows that the Shibandun mafic-ultramafic cumulates were likely co-magmatic, and the magma was likely derived from the subduction fluid-metasomatized asthenospheric mantle.The magmatic evolution process may have involved repeated cycles of magma fractionation, cumulate formation and basaltic magma replenishment.The Shibandunmafic-ultramafic cumulates were likely formed in volcanic arc setting.Integrated with the regional tectonic background, we propose that the western part of the Central Qilian microcontinent was likely a volcanic arc accretionary complex that was formed on the residual microcontinental margin.

Key words: cumulate, Shibandun, western part of the Central Qilian, Early Paleozoic, volcanic arc accretionary complex

CLC Number:

P588.1

MA Zhen, LIU Zheng, LIU Yixin, WANG Jinrong. Geochemical Characteristics and Tectonic Significance of Shibandun Mafic-ultramafic Cumulates in Western Part of the Central Qilian[J]. Geoscience, 2018, 32(04): 667-679.

Figures/Tables 10

References 65

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样品号及分析点号	含量/10^-6			Th/U	同位素比值					年龄/Ma
样品号及分析点号	Pb	Th	U	Th/U	²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ		²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ
DLT-11-19-01	414.07	461.06	1 089.47	0.423 196 600	0.607 85	0.013 12	0.079 38	0.000 60	482.2		8.28	492.4	3.61
DLT-11-19-02	1 341.13	2 565.25	3 346.19	0.766 618 154	0.639 02	0.006 72	0.078 96	0.000 44	501.7		4.16	489.9	2.65
DLT-11-19-03	920.49	1 844.66	2 384.47	0.773 614 262	0.609 76	0.005 27	0.078 65	0.000 42	483.4		3.33	488.1	2.51
DLT-11-19-04	1 797.78	3 604.71	3 914.26	0.920 917 364	0.601 61	0.008 89	0.076 48	0.000 48	478.3		5.64	475.1	2.87
DLT-11-19-05	577.79	545.93	1 537.84	0.354 997 919	0.653 90	0.011 40	0.078 03	0.000 53	510.9		7.00	484.4	3.20
DLT-11-19-06	352.83	303.24	939.56	0.322 746 818	0.624 11	0.007 45	0.078 76	0.000 46	492.4		4.66	488.7	2.72
DLT-11-19-07	237.25	66.79	91.35	0.731 143 952	9.727 39	0.119 93	0.450 67	0.003 77	2 409.3		11.35	2 398.2	16.76
DLT-11-19-08	95.59	51.23	264.85	0.193 430 244	0.631 23	0.043 12	0.078 95	0.001 51	496.9		26.84	489.8	9.03
DLT-11-19-09	469.61	559.9	1 113.61	0.502 779 249	0.670 47	0.014 78	0.078 33	0.000 61	521.0		8.98	486.1	3.66
DLT-11-19-11	420.95	899.25	1 121.43	0.801 877 959	0.646 45	0.010 68	0.078 34	0.000 52	506.3		6.59	486.2	3.08
DLT-11-19-13	1 813.43	314.2	738.33	0.425 554 969	9.689 35	0.060 31	0.452 33	0.002 48	2 405.7		5.73	2 405.6	10.99
DLT-11-19-21	641.77	698.2	1 772.03	0.394 011 388	0.637 36	0.007 22	0.078 15	0.000 44	500.7		4.48	485.0	2.65
DLT-11-19-22	8 021.48	716.09	2 524.88	0.283 613 479	0.670 82	0.030 87	0.076 78	0.001 05	521.2		18.76	476.9	6.30
DLT-11-19-23	695.34	947.97	1 650.90	0.574 214 065	0.634 09	0.016 09	0.079 81	0.000 66	498.6		10.00	495.0	3.95
DLT-11-19-24	597.66	143.29	182.50	0.785 150 685	13.102 05	0.123 61	0.517 14	0.003 65	2 687.0		8.90	2 687.0	15.50

样品号及分析点号	含量/10^-6			Th/U	同位素比值					年龄/Ma
样品号及分析点号	Pb	Th	U	Th/U	²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ		²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ
DLT-11-19-01	414.07	461.06	1 089.47	0.423 196 600	0.607 85	0.013 12	0.079 38	0.000 60	482.2		8.28	492.4	3.61
DLT-11-19-02	1 341.13	2 565.25	3 346.19	0.766 618 154	0.639 02	0.006 72	0.078 96	0.000 44	501.7		4.16	489.9	2.65
DLT-11-19-03	920.49	1 844.66	2 384.47	0.773 614 262	0.609 76	0.005 27	0.078 65	0.000 42	483.4		3.33	488.1	2.51
DLT-11-19-04	1 797.78	3 604.71	3 914.26	0.920 917 364	0.601 61	0.008 89	0.076 48	0.000 48	478.3		5.64	475.1	2.87
DLT-11-19-05	577.79	545.93	1 537.84	0.354 997 919	0.653 90	0.011 40	0.078 03	0.000 53	510.9		7.00	484.4	3.20
DLT-11-19-06	352.83	303.24	939.56	0.322 746 818	0.624 11	0.007 45	0.078 76	0.000 46	492.4		4.66	488.7	2.72
DLT-11-19-07	237.25	66.79	91.35	0.731 143 952	9.727 39	0.119 93	0.450 67	0.003 77	2 409.3		11.35	2 398.2	16.76
DLT-11-19-08	95.59	51.23	264.85	0.193 430 244	0.631 23	0.043 12	0.078 95	0.001 51	496.9		26.84	489.8	9.03
DLT-11-19-09	469.61	559.9	1 113.61	0.502 779 249	0.670 47	0.014 78	0.078 33	0.000 61	521.0		8.98	486.1	3.66
DLT-11-19-11	420.95	899.25	1 121.43	0.801 877 959	0.646 45	0.010 68	0.078 34	0.000 52	506.3		6.59	486.2	3.08
DLT-11-19-13	1 813.43	314.2	738.33	0.425 554 969	9.689 35	0.060 31	0.452 33	0.002 48	2 405.7		5.73	2 405.6	10.99
DLT-11-19-21	641.77	698.2	1 772.03	0.394 011 388	0.637 36	0.007 22	0.078 15	0.000 44	500.7		4.48	485.0	2.65
DLT-11-19-22	8 021.48	716.09	2 524.88	0.283 613 479	0.670 82	0.030 87	0.076 78	0.001 05	521.2		18.76	476.9	6.30
DLT-11-19-23	695.34	947.97	1 650.90	0.574 214 065	0.634 09	0.016 09	0.079 81	0.000 66	498.6		10.00	495.0	3.95
DLT-11-19-24	597.66	143.29	182.50	0.785 150 685	13.102 05	0.123 61	0.517 14	0.003 65	2 687.0		8.90	2 687.0	15.50

样号	岩性		SiO₂		TiO₂		Al₂O₃		TFe₂O₃		MnO		MgO		CaO		Na₂O		K₂O		P₂O₅		烧失量		总量	Mg^#		Rb		Ba
YQ-29	蛇纹石化橄辉岩		45.82		0.15		4.76		10.31		0.14		31.23		2.21		0.42		0.26		0.03		4.23		99.57	72.19		12.61		43.48
YQ-30			47.93		0.08		5.70		8.40		0.13		28.95		2.71		0.15		0.47		0.01		5.04		99.58	74.72		20.08		84.46
YQ-31			54.93		0.18		0.62		9.97		0.14		23.43		6.58		0.57		0.08		0.03		3.03		99.56	66.83		3.06		44.28
YQ-28	辉长岩		49.02		0.29		18.89		4.76		0.08		9.55		13.88		1.68		0.17		0.03		1.19		99.56	63.22		4.74		77.66
DLT-11-19			46.12		0.29		16.30		6.38		0.10		12.98		11.75		1.45		0.43		0.03		3.69		99.52	63.56		9.82		39.40
DLT-11-21			47.47		0.15		17.98		5.01		0.08		10.96		13.33		1.60		0.11		0.01		2.85		99.55	65.22		3.59		56.25
DLT-11-23			43.25		0.08		16.06		7.28		0.09		17.49		9.18		0.85		0.22		0.02		5.02		99.54	67.32		8.44		43.91
样号	岩性		Th		U		Nb		Ta		La		Ce		Pb		Pr		Sr		Nd		Sm		Zr	Hf		Eu		Gd
YQ-29	蛇纹石化橄辉岩		0.71		0.29		1.17		0.11		2.25		4.24		4.02		0.51		72.30		1.95		0.44		14.31	0.33		0.23		0.47
YQ-30			0.12		0.07		0.39		0.06		0.87		1.62		5.61		0.19		51.40		0.70		0.15		5.38	0.14		0.16		0.17
YQ-31			0.24		0.15		0.55		0.06		1.43		3.26		1.84		0.46		106.02		2.11		0.60		14.06	0.33		0.21		0.73
YQ-28	辉长岩		0.33		0.14		1.39		0.17		2.38		5.09		2.28		0.72		306.94		3.22		0.89		19.51	0.46		0.64		1.10
DLT-11-19			0.29		0.09		0.91		0.06		1.79		3.84		1.36		0.52		164.20		2.40		0.63		11.01	0.31		0.34		0.76
DLT-11-21			0.09		0.05		0.25		0.02		1.22		2.72		1.42		0.40		316.88		1.97		0.61		5.91	0.21		0.66		0.78
DLT-11-23			0.77		0.35		0.48		0.04		1.81		3.51		2.14		0.42		187.33		1.74		0.43		22.62	0.66		0.39		0.50
样号	岩性	Tb		Dy		Ho		Y		Er		Tm		Yb		Lu		ΣREE		ΣLREE		δEu		(La/Yb)_N			(La/Sm)_N		(Gd/Yb)_N
YQ-29	蛇纹石化橄辉岩	0.07		0.47		0.09		3.33		0.29		0.04		0.31		0.05		11.42		9.62		1.53		4.87			3.20		1.23
YQ-30		0.02		0.18		0.04		1.34		0.13		0.02		0.16		0.02		4.42		3.68		3.04		3.59			3.76		0.82
YQ-31		0.12		0.80		0.16		5.05		0.47		0.07		0.45		0.06		10.92		8.07		0.97		2.16			1.51		1.33
YQ-28	辉长岩	0.18		1.16		0.23		7.78		0.55		0.09		0.60		0.08		16.93		12.94		1.98		2.69			1.68		1.49
DLT-11-19		0.12		0.75		0.16		4.13		0.43		0.06		0.40		0.06		12.28		9.54		1.51		3.05			1.78		1.54
DLT-11-21		0.13		0.86		0.18		4.56		0.49		0.07		0.44		0.06		10.59		7.58		2.91		1.89			1.27		1.44
DLT-11-23		0.08		0.51		0.10		2.79		0.31		0.05		0.31		0.05		10.21		8.30		2.59		3.91			2.64		1.28

样号	岩性		SiO₂		TiO₂		Al₂O₃		TFe₂O₃		MnO		MgO		CaO		Na₂O		K₂O		P₂O₅		烧失量		总量	Mg^#		Rb		Ba
YQ-29	蛇纹石化橄辉岩		45.82		0.15		4.76		10.31		0.14		31.23		2.21		0.42		0.26		0.03		4.23		99.57	72.19		12.61		43.48
YQ-30			47.93		0.08		5.70		8.40		0.13		28.95		2.71		0.15		0.47		0.01		5.04		99.58	74.72		20.08		84.46
YQ-31			54.93		0.18		0.62		9.97		0.14		23.43		6.58		0.57		0.08		0.03		3.03		99.56	66.83		3.06		44.28
YQ-28	辉长岩		49.02		0.29		18.89		4.76		0.08		9.55		13.88		1.68		0.17		0.03		1.19		99.56	63.22		4.74		77.66
DLT-11-19			46.12		0.29		16.30		6.38		0.10		12.98		11.75		1.45		0.43		0.03		3.69		99.52	63.56		9.82		39.40
DLT-11-21			47.47		0.15		17.98		5.01		0.08		10.96		13.33		1.60		0.11		0.01		2.85		99.55	65.22		3.59		56.25
DLT-11-23			43.25		0.08		16.06		7.28		0.09		17.49		9.18		0.85		0.22		0.02		5.02		99.54	67.32		8.44		43.91
样号	岩性		Th		U		Nb		Ta		La		Ce		Pb		Pr		Sr		Nd		Sm		Zr	Hf		Eu		Gd
YQ-29	蛇纹石化橄辉岩		0.71		0.29		1.17		0.11		2.25		4.24		4.02		0.51		72.30		1.95		0.44		14.31	0.33		0.23		0.47
YQ-30			0.12		0.07		0.39		0.06		0.87		1.62		5.61		0.19		51.40		0.70		0.15		5.38	0.14		0.16		0.17
YQ-31			0.24		0.15		0.55		0.06		1.43		3.26		1.84		0.46		106.02		2.11		0.60		14.06	0.33		0.21		0.73
YQ-28	辉长岩		0.33		0.14		1.39		0.17		2.38		5.09		2.28		0.72		306.94		3.22		0.89		19.51	0.46		0.64		1.10
DLT-11-19			0.29		0.09		0.91		0.06		1.79		3.84		1.36		0.52		164.20		2.40		0.63		11.01	0.31		0.34		0.76
DLT-11-21			0.09		0.05		0.25		0.02		1.22		2.72		1.42		0.40		316.88		1.97		0.61		5.91	0.21		0.66		0.78
DLT-11-23			0.77		0.35		0.48		0.04		1.81		3.51		2.14		0.42		187.33		1.74		0.43		22.62	0.66		0.39		0.50
样号	岩性	Tb		Dy		Ho		Y		Er		Tm		Yb		Lu		ΣREE		ΣLREE		δEu		(La/Yb)_N			(La/Sm)_N		(Gd/Yb)_N
YQ-29	蛇纹石化橄辉岩	0.07		0.47		0.09		3.33		0.29		0.04		0.31		0.05		11.42		9.62		1.53		4.87			3.20		1.23
YQ-30		0.02		0.18		0.04		1.34		0.13		0.02		0.16		0.02		4.42		3.68		3.04		3.59			3.76		0.82
YQ-31		0.12		0.80		0.16		5.05		0.47		0.07		0.45		0.06		10.92		8.07		0.97		2.16			1.51		1.33
YQ-28	辉长岩	0.18		1.16		0.23		7.78		0.55		0.09		0.60		0.08		16.93		12.94		1.98		2.69			1.68		1.49
DLT-11-19		0.12		0.75		0.16		4.13		0.43		0.06		0.40		0.06		12.28		9.54		1.51		3.05			1.78		1.54
DLT-11-21		0.13		0.86		0.18		4.56		0.49		0.07		0.44		0.06		10.59		7.58		2.91		1.89			1.27		1.44
DLT-11-23		0.08		0.51		0.10		2.79		0.31		0.05		0.31		0.05		10.21		8.30		2.59		3.91			2.64		1.28