Characteristics and Geological Significance of Thick Ignimbrite Beds of Yanchang Formation (Chang 7 Section) in Southeastern Ordos Basin

doi:10.19657/j.geoscience.1000-8527.2019.02.14

Abstract

Abstract:

Ignimbrite interbeds are widely distributed in the high-grade hydrocarbon source rocks in the Chang 7 member of the Yanchang Formation in the Ordos basin. Generally, a single layer is 0.1 to 10 cm thick, occasionally reaching 1 to 2 m. Field geological and microscope petrographic studies at Yaoquzhen (Tongchuan city, southern Ordos basin) have identified a ~14-meter thick lithic crystal ignimbrite with interbedded lensoidal carbonatite. The SiO₂ content is of 47.09% to 66.6%, which show that the rocks are primarily intermediate-felsic. Total REE (∑REE) contents are 164.04×10^-6 to 245.74×10^-6, and the rocks are enriched in LREEs, depleted in HREEs, with right-inclining REE pattern and negative Eu anomalies. LILEs (such as Sr and P) show negative anomalies whereas HFSEs (such as U and Th) show positive anomalies. For the carbonatites, the relatively high SiO₂ and simple mineral assemblage suggest a crustal origin, whilst the high Sr and Ba contents show an igneous origin. Based on the geological and petrographic features of the ignimbrite and their distribution characteristics, the Middle-Late Triassic volcanic lithofacies model in the Ordos basin is established and the ignimbrite is likely of volcanic feeder-explosive facies. The ignimbrite was primarily from volcanic arc and had a high-K calc-alkaline magma source. The lensoidal carbonatites may have formed during the extrusion along extensional lithospheric faults resulted from continental collision, with the carbonatites produced directly from low-degree partial melting of the lithospheric mantle, which was intimately related to the evolution of Qinling orogenic belt that was responsible to mountain-basin coupling. The discovery provides a new petrological basis to understand the tectonic evolution of the Mesozoic Ordos basin.

Key words: ignimbrite, carbonatite, volcanic feeder-explosive facies, volcanic lithofacies, Yaoquzhen Tongchuan city, Yanchang Formation, Ordos basin

CLC Number:

TE121

XU Feng, ZHU Zengwu, LI Changchun, YANG Zhiguo. Characteristics and Geological Significance of Thick Ignimbrite Beds of Yanchang Formation (Chang 7 Section) in Southeastern Ordos Basin[J]. Geoscience, 2019, 33(02): 389-400.

Figures/Tables 11

Fig.1 Simplified tectonic map of the Qinling orogenic belt ((a) modified after Yang[10], 2004) and distribution of Ordos basin and other peripheral basins ((b) modified after Changqing Oilfield[11], 1992)

Table 1 Evolution of stratigraphic division of the Triassic Yanchang Formation in the Ordos basin

王竹泉和潘钟祥 (1933)		甘克文^① (1959)		谢秋文等^② (1974)		陕西省地质矿产局(1989)		全国地层委员会 (2002)		陕西区域地质志 (2013)		长庆油田 (2009)
侏罗系	瓦窑堡煤系	上三叠统延长组	T₃y⁵	上三叠统延长组	T₃ $y 5 1 - 5$	上三叠统	瓦窑堡组 ( $T 3 3$ )	上三叠统	瓦窑堡阶 ( $T 3 3$ )	上三叠统	瓦窑堡组 (T₃w)	上三叠统	长1
上三叠统	延长组		T₃y⁴		T₃y⁴		永平组 ( $T 3 2$ )		永平阶 ( $T 3 2$ )		(T_2-3y⁴)		长2
			T₃y⁴		T₃y⁴		永平组 ( $T 3 2$ )		永平阶 ( $T 3 2$ )		(T_2-3y⁴)		长3
			T₃y³		T₃ $y 3 1 - 3$		胡家村组 ( $T 3 1$ )		胡家村阶 ( $T 3 1$ )		(T_2-3y³)		长4+5
			T₃y³		T₃ $y 3 1 - 3$		胡家村组 ( $T 3 1$ )		胡家村阶 ( $T 3 1$ )				长6
			T₃y²		T₃ $y 2 1 - 2$	中三叠统	铜川组 ( $T 2 2$ )	中三叠统	铜川阶 ( $T 2 2$ )	中三叠统			长7
											(T_2-3y²)	中三叠统	长8
											(T_2-3y²)		长9
			T₃y¹		T₃y¹						(T_2-3y¹)		长10

Table 1 Evolution of stratigraphic division of the Triassic Yanchang Formation in the Ordos basin

王竹泉和潘钟祥 (1933)		甘克文^① (1959)		谢秋文等^② (1974)		陕西省地质矿产局(1989)		全国地层委员会 (2002)		陕西区域地质志 (2013)		长庆油田 (2009)
侏罗系	瓦窑堡煤系	上三叠统延长组	T₃y⁵	上三叠统延长组	T₃ $y 5 1 - 5$	上三叠统	瓦窑堡组 ( $T 3 3$ )	上三叠统	瓦窑堡阶 ( $T 3 3$ )	上三叠统	瓦窑堡组 (T₃w)	上三叠统	长1
上三叠统	延长组		T₃y⁴		T₃y⁴		永平组 ( $T 3 2$ )		永平阶 ( $T 3 2$ )		(T_2-3y⁴)		长2
			T₃y⁴		T₃y⁴		永平组 ( $T 3 2$ )		永平阶 ( $T 3 2$ )		(T_2-3y⁴)		长3
			T₃y³		T₃ $y 3 1 - 3$		胡家村组 ( $T 3 1$ )		胡家村阶 ( $T 3 1$ )		(T_2-3y³)		长4+5
			T₃y³		T₃ $y 3 1 - 3$		胡家村组 ( $T 3 1$ )		胡家村阶 ( $T 3 1$ )				长6
			T₃y²		T₃ $y 2 1 - 2$	中三叠统	铜川组 ( $T 2 2$ )	中三叠统	铜川阶 ( $T 2 2$ )	中三叠统			长7
											(T_2-3y²)	中三叠统	长8
											(T_2-3y²)		长9
			T₃y¹		T₃y¹						(T_2-3y¹)		长10

Fig.2 Outcrop photos and microphotographs of the ignimbrite

Fig.3 Lithostratigraphic column and sampling horizon of the ignimbrite at Yaoquzhen, Tongchuan city

Fig.4 Lithostratigraphic comparison of the Yanchang Formation ignimbrite in the southern Ordos basin

Fig.5 Sketch map of Middle-Late Triassic volcanic lithofacies in the Ordos basin

Table 2 Whole-rock chemical composition (%), REE and trace elements (10-6) of the ignimbrite and carbonatites

样号	SiO₂	Al₂O₃	TFe₂O₃	MgO	CaO	Na₂O	K₂O	Cr₂O₃	P₂O₅	SO₃	MnO	TiO₂	烧失量	Total	Rb	Sr
熔结凝灰岩1	66.49	12.71	2.48	0.47	2.94	2.98	1.88	0.003	0.05	0.87	0.30	0.17	8.56	99.90	119	136
熔结凝灰岩2	60.96	14.77	4.14	0.78	0.93	1.09	3.84	0.001	0.15	1.98	0.11	0.29	10.10	99.14	238	119
熔结凝灰岩3	66.60	13.04	2.62	0.62	1.13	3.07	3.09	0.003	0.10	1.29	0.08	0.24	7.70	99.58	157	113
熔结凝灰岩4	66.22	12.82	2.69	0.80	1.27	1.89	3.55	0.003	0.10	1.85	0.09	0.25	8.10	99.63	181	123
熔结凝灰岩5	47.09	10.20	5.09	5.29	7.93	1.33	2.76	0.01	0.10	2.48	0.19	0.24	17.25	99.96	114	160
凝灰岩6	55.57	16.04	5.42	1.94	1.81	0.69	3.19	0.01	0.15	2.90	0.09	0.69	10.68	99.18	146	138
碳酸岩1	31.35	6.69	5.25	3.05	23.66	1.43	0.87	0.0013	0.88	0.74	0.23	0.13	26.19	100.47	42.4	975
碳酸岩2	31.95	5.94	5.40	3.25	23.24	1.12	0.87	0.0010	0.62	0.46	0.23	0.11	26.75	99.94	40.5	818
碳酸岩3	22.85	5.80	4.73	3.22	29.52	1.06	0.87	0.0003	0.67	0.47	0.24	0.06	30.46	99.95	41.0	999
碳酸岩4	47.65	6.96	6.33	3.36	12.96	0.91	1.34	0.0016	0.64	0.79	0.25	0.13	19.08	100.40	69.9	351
样号	Ba	Th	U	Nb	P	Zr	Hf	Sc	Ni	Ta	F	V	Cr	La	Ce	Pr
熔结凝灰岩1	420	23.7	7.24	19.8	300	136	7.50	5.90	29.4	1.76	805	34.8	19.6	49.7	99.9	11.9
熔结凝灰岩2	624	26.9	15.8	15.2	780	131	5.68	6.90	17.7	2.18	841	69.6	30.5	43.8	78.8	8.71
熔结凝灰岩3	507	27.7	12.0	18.0	485	134	6.52	6.30	78.1	1.94	660	43.9	17.3	38.7	70.1	7.32
熔结凝灰岩4	497	26.1	10.3	14.5	471	141	6.05	5.30	17.1	1.95	722	42.8	16.7	42.1	75.6	8.33
熔结凝灰岩5	460	22.4	8.82	13.7	480	131	5.63	9.10	8.77	1.50	617	78.6	27.8	39.5	69.8	7.81
凝灰岩6	446	17.1	5.33	14.8	668	166	6.80	12.4	36.3	1.42	736	133	81.4	54.4	102	11.5
碳酸岩1	232	12.5	6.48	16.2	3673	137	3.49	6.5	11.7	0.74	1333	75.1	16.6	26.9	53.2	6.70
碳酸岩2	252	13.6	5.57	3.85	2705	124	0.80	6.6	11.2	0.76	1112	65.4	13.2	19.4	36.4	4.81
碳酸岩3	205	15.7	6.43	3.65	3464	112	0.87	6.3	9.88	0.52	1062	58.7	8.3	19.7	33.1	4.11
碳酸岩4	297	14.5	7.60	4.99	2426	128	0.76	9.3	10.1	0.51	1172	66.8	14.7	23.1	42.7	5.39
样号	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	ΣREE	LREE	HREE	LREE/ HREE
熔结凝灰岩1	43.4	9.10	0.83	8.50	1.35	8.13	1.84	4.63	0.79	4.86	0.81	54.4	245.74	214.83	30.91	6.95
熔结凝灰岩2	30.0	5.91	1.12	5.96	0.89	4.87	1.12	3.15	0.50	3.30	0.59	33.9	188.72	168.34	20.38	8.26
熔结凝灰岩3	25.4	4.86	0.88	4.58	0.78	3.90	0.82	2.55	0.48	3.17	0.50	29.0	164.04	147.26	16.78	8.78
熔结凝灰岩4	29.0	5.38	0.94	5.40	0.84	4.42	0.99	2.79	0.48	3.44	0.52	31.5	180.23	161.35	18.88	8.55
熔结凝灰岩5	25.9	5.23	0.90	5.30	0.71	3.90	0.96	2.78	0.50	3.48	0.52	31.2	167.29	149.14	18.15	8.22
凝灰岩6	42.4	7.79	1.75	7.00	1.06	5.29	1.09	2.92	0.43	2.86	0.41	32.4	240.90	219.84	21.06	10.44
碳酸岩1	26.4	6.47	0.93	6.36	1.42	6.68	1.93	4.60	0.91	5.26	0.95	57.5	148.71	120.60	28.11	4.29
碳酸岩2	18.1	4.15	0.63	4.40	0.90	4.63	1.26	3.21	0.65	4.12	0.74	40.0	103.4	83.49	19.91	4.19
碳酸岩3	15.0	3.33	0.57	3.75	0.82	4.35	1.37	3.63	0.77	4.67	0.89	46.1	96.06	75.81	20.25	3.74
碳酸岩4	20.5	4.86	0.89	4.85	1.20	5.79	1.74	4.65	0.88	5.87	1.06	53.2	123.48	97.44	26.04	3.74

Table 2 Whole-rock chemical composition (%), REE and trace elements (10-6) of the ignimbrite and carbonatites

样号	SiO₂	Al₂O₃	TFe₂O₃	MgO	CaO	Na₂O	K₂O	Cr₂O₃	P₂O₅	SO₃	MnO	TiO₂	烧失量	Total	Rb	Sr
熔结凝灰岩1	66.49	12.71	2.48	0.47	2.94	2.98	1.88	0.003	0.05	0.87	0.30	0.17	8.56	99.90	119	136
熔结凝灰岩2	60.96	14.77	4.14	0.78	0.93	1.09	3.84	0.001	0.15	1.98	0.11	0.29	10.10	99.14	238	119
熔结凝灰岩3	66.60	13.04	2.62	0.62	1.13	3.07	3.09	0.003	0.10	1.29	0.08	0.24	7.70	99.58	157	113
熔结凝灰岩4	66.22	12.82	2.69	0.80	1.27	1.89	3.55	0.003	0.10	1.85	0.09	0.25	8.10	99.63	181	123
熔结凝灰岩5	47.09	10.20	5.09	5.29	7.93	1.33	2.76	0.01	0.10	2.48	0.19	0.24	17.25	99.96	114	160
凝灰岩6	55.57	16.04	5.42	1.94	1.81	0.69	3.19	0.01	0.15	2.90	0.09	0.69	10.68	99.18	146	138
碳酸岩1	31.35	6.69	5.25	3.05	23.66	1.43	0.87	0.0013	0.88	0.74	0.23	0.13	26.19	100.47	42.4	975
碳酸岩2	31.95	5.94	5.40	3.25	23.24	1.12	0.87	0.0010	0.62	0.46	0.23	0.11	26.75	99.94	40.5	818
碳酸岩3	22.85	5.80	4.73	3.22	29.52	1.06	0.87	0.0003	0.67	0.47	0.24	0.06	30.46	99.95	41.0	999
碳酸岩4	47.65	6.96	6.33	3.36	12.96	0.91	1.34	0.0016	0.64	0.79	0.25	0.13	19.08	100.40	69.9	351
样号	Ba	Th	U	Nb	P	Zr	Hf	Sc	Ni	Ta	F	V	Cr	La	Ce	Pr
熔结凝灰岩1	420	23.7	7.24	19.8	300	136	7.50	5.90	29.4	1.76	805	34.8	19.6	49.7	99.9	11.9
熔结凝灰岩2	624	26.9	15.8	15.2	780	131	5.68	6.90	17.7	2.18	841	69.6	30.5	43.8	78.8	8.71
熔结凝灰岩3	507	27.7	12.0	18.0	485	134	6.52	6.30	78.1	1.94	660	43.9	17.3	38.7	70.1	7.32
熔结凝灰岩4	497	26.1	10.3	14.5	471	141	6.05	5.30	17.1	1.95	722	42.8	16.7	42.1	75.6	8.33
熔结凝灰岩5	460	22.4	8.82	13.7	480	131	5.63	9.10	8.77	1.50	617	78.6	27.8	39.5	69.8	7.81
凝灰岩6	446	17.1	5.33	14.8	668	166	6.80	12.4	36.3	1.42	736	133	81.4	54.4	102	11.5
碳酸岩1	232	12.5	6.48	16.2	3673	137	3.49	6.5	11.7	0.74	1333	75.1	16.6	26.9	53.2	6.70
碳酸岩2	252	13.6	5.57	3.85	2705	124	0.80	6.6	11.2	0.76	1112	65.4	13.2	19.4	36.4	4.81
碳酸岩3	205	15.7	6.43	3.65	3464	112	0.87	6.3	9.88	0.52	1062	58.7	8.3	19.7	33.1	4.11
碳酸岩4	297	14.5	7.60	4.99	2426	128	0.76	9.3	10.1	0.51	1172	66.8	14.7	23.1	42.7	5.39
样号	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	ΣREE	LREE	HREE	LREE/ HREE
熔结凝灰岩1	43.4	9.10	0.83	8.50	1.35	8.13	1.84	4.63	0.79	4.86	0.81	54.4	245.74	214.83	30.91	6.95
熔结凝灰岩2	30.0	5.91	1.12	5.96	0.89	4.87	1.12	3.15	0.50	3.30	0.59	33.9	188.72	168.34	20.38	8.26
熔结凝灰岩3	25.4	4.86	0.88	4.58	0.78	3.90	0.82	2.55	0.48	3.17	0.50	29.0	164.04	147.26	16.78	8.78
熔结凝灰岩4	29.0	5.38	0.94	5.40	0.84	4.42	0.99	2.79	0.48	3.44	0.52	31.5	180.23	161.35	18.88	8.55
熔结凝灰岩5	25.9	5.23	0.90	5.30	0.71	3.90	0.96	2.78	0.50	3.48	0.52	31.2	167.29	149.14	18.15	8.22
凝灰岩6	42.4	7.79	1.75	7.00	1.06	5.29	1.09	2.92	0.43	2.86	0.41	32.4	240.90	219.84	21.06	10.44
碳酸岩1	26.4	6.47	0.93	6.36	1.42	6.68	1.93	4.60	0.91	5.26	0.95	57.5	148.71	120.60	28.11	4.29
碳酸岩2	18.1	4.15	0.63	4.40	0.90	4.63	1.26	3.21	0.65	4.12	0.74	40.0	103.4	83.49	19.91	4.19
碳酸岩3	15.0	3.33	0.57	3.75	0.82	4.35	1.37	3.63	0.77	4.67	0.89	46.1	96.06	75.81	20.25	3.74
碳酸岩4	20.5	4.86	0.89	4.85	1.20	5.79	1.74	4.65	0.88	5.87	1.06	53.2	123.48	97.44	26.04	3.74

Fig.6 K2O vs. SiO2 plot for the ignimbrite (solid-line after Peccerillo and Talyer[16], 1976; dash-line after Middlimost, 1985)

Fig.7 Chondrite-normalized REE patterns for the ignimbrite (a) and carbonatites (b) (chondrite normalizing values after Sun and McDonough[17], 1989)

Fig.8 Primitive mantle (PM)-normalized spider diagram for the ignimbrite (a) and carbonatites (b)(PM normalizing values after Sun and Mc Donough[17], 1989)

Fig.9 Tectonic discrimination diagrams for the Yanchang Formation ignimbrite

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