四川矿山梁子苦橄玢岩成因:来自橄榄石成分剖面的约束

doi:10.13745/j.esf.sf.2025.7.27

摘要/Abstract

摘要：

苦橄岩通常作为地幔柱活动的证据之一,得到了众多学者的关注。然而,许多苦橄岩中含有不同结晶习性的橄榄石晶体,暗示了外来橄榄石晶体的加入。因此,阐明苦橄岩中橄榄石晶体的性质是理解苦橄岩成因及其地质意义的关键环节。本文报道了川西矿山梁子苦橄玢岩岩席中代表性橄榄石颗粒的成分剖面分析结果,发现苦橄玢岩含有4类橄榄石晶体,分别将其称为粗晶橄榄石、聚斑状橄榄石、包裹体橄榄石和鬣刺状橄榄石。统计分析表明粗晶橄榄石具有一个Fo₈₄的主要峰值和一个Fo₇₇的次要峰值;聚斑状橄榄石具有一个Fo₉₂的主要峰值和一个Fo₇₇的次要峰值;包裹体橄榄石(Fo₈₂)和鬣刺状橄榄石(Fo₇₇)只有一个峰值,其中包裹体橄榄石具有均一的成分,而鬣刺状橄榄石具有陡的成分梯度。将成分相近的粗晶橄榄石和包裹体橄榄石合并,则矿山梁子苦橄玢岩中包含3种Fo分布范围明显不同的橄榄石晶体群。这3种晶体群也产于峨眉山玄武岩和大板山辉长岩中,因而可以认为岩浆混合作用是峨眉山大火成岩省岩浆系统的遍在性岩浆过程。热力学和流体动力学分析揭示,聚斑状橄榄石形成于约4 GPa和约1 600 ℃,粗晶橄榄石(及包裹体橄榄石)主要形成于约2~2.5 GPa和约1 450~1 550 ℃,而鬣刺状橄榄石形成于约0.1 GPa和约1 200 ℃。据此,可以认为鬣刺状橄榄石形成于携带岩浆的结晶作用,另两类橄榄石则是携带岩浆从深部岩浆房中捕获的循环晶(antecryst)。橄榄石的NiO含量总体随Fo增加而增加,但存在4个不同的次级变异趋势,其特点是Fo值增加不大的背景下NiO含量急剧升高,暗示岩浆为挥发分流体过饱和系统。此外,样品的橄榄石体积分数随高程强烈变化,与聚斑状橄榄石的产出一起,暗示了岩浆的快速运动。据此,本文认为矿山梁子岩浆系统为至少具有3个深部岩浆房的系统,苦橄玢岩是熔体、晶体和流体的混合物,而不是原生苦橄质岩浆固结的产物。

关键词: 平川铁矿, 苦橄质岩床, 橄榄石晶体群, 透岩浆流体, 攀枝花式铁矿

Abstract:

Picrite, as one of the evident manifestations of mantle plume activity, has garnered significant attention from researchers. However, many picrites contain olivine crystals exhibiting diverse crystallization habits, indicating the incorporation of xenocrystic olivine. Thus, characterizing the properties of olivine crystals is crucial for understanding the petrogenesis of picrites and their geological implications. This study presents compositional profile analyses of representative olivine grains from picrite sills in the Kuangshanliangzi Fe deposit, western Sichuan Province. We identify four distinct populations of olivine crystals: coarse-grained olivine, aggregate olivine, inclusion olivine, and spinifex olivine. Statistical analyses reveal that coarse-grained olivine exhibits a major peak at ~Fo₈₄ and a minor peak at ~Fo₇₇. Aggregate olivine shows a major peak at ~Fo₉₂ and a minor peak at ~Fo₇₇. Inclusion olivine (~Fo₈₂) and spinifex olivine (~Fo₇₇) each display a single peak, with the former being compositionally uniform and the latter exhibiting a steep compositional gradient. By combining compositionally similar coarse-grained and inclusion olivines, three distinct olivine populations are recognized in the Kuangshanliangzi picrites. These three types are also identified in Emeishan basalt and Dabanshan gabbro, suggesting that magma mixing is a ubiquitous process within the magmatic system of the Emeishan Large Igneous Province. Thermodynamic and fluid dynamic analyses indicate that aggregate olivine crystallized at ~4 GPa and ~1600 ℃, coarse-grained olivine (and inclusion olivine) mainly formed at ~2-2.5 GPa and ~1450-1550 ℃, and spinifex olivine crystallized at ~0.1 GPa and ~1200 ℃. Accordingly, spinifex olivine is interpreted to have crystallized from the carrier magma, whereas the other two types represent antecrysts transported from deeper magma chambers by the ascending melt. The NiO content of olivine generally increases with Fo content but exhibits four distinct secondary trends. These trends are characterized by modest increases in Fo accompanied by sharp rises in NiO, implying that the magma was a volatile-fluid-supersaturated system. Furthermore, the volumetric proportion of olivine varies markedly with elevation, and the occurrence of aggregate olivine suggests rapid magma ascent. Based on these findings, we propose that the Kuangshanliangzi magmatic system comprised at least three deep-seated magma chambers, and that the picrites represent mixtures of melt, crystals, and fluids rather than products of direct consolidation from a primitive picritic magma.

Key words: Pingchuan iron deposit, picrite sill, olivine crystal population, Transmagmatic fluid, Panzhihua Fe deposit

中图分类号:

罗照华. 四川矿山梁子苦橄玢岩成因:来自橄榄石成分剖面的约束[J]. 地学前缘, 2025, 32(6): 286-302.

LUO Zhaohua. Genesis of picrites in Sichuan Kuangshanliangzi: Constraints from olivine composition profile[J]. Earth Science Frontiers, 2025, 32(6): 286-302.

图/表 16

图1 研究区地理位置图(a)、平川地区区域地质图(b)和矿山梁子矿区地质图(c)(图1b据文献[18]修编)

Fig.1 (a) Geographical location of the study area; (b) Regional geologyical map of Pingchuan (modified after [18]); (c) The mining geological map of Kuangshanliangzi

图2 矿山梁子四号苦橄玢岩岩席的产状

Fig.2 Occurrence of the picrite sill No.4 in the Kuangshanliangzi ore district

图3 四号岩席苦橄玢岩的显微照片(正交偏光) Ol—橄榄石;Cpx—单斜辉石;Pl—斜长石; Mt—磁铁矿。

Fig.3 A microphotograph of the picrite sill No.4 (cross-polarized light)

图4 四号岩席苦橄玢岩中的4类代表性橄榄石晶体特征 a—粗晶橄榄石;b—聚斑状橄榄石;c—“鬣刺”状橄榄石;d—包裹体橄榄石。Ol—橄榄石;Cpx—单斜辉石。

Fig.4 Characteristics of the four types of representative olivines from the picrite sill No.4

图5 橄榄石斑晶体积分数(a)和全岩MgO含量(b)与样品分布高程的关系

Fig.5 Variations of the volume fraction of olivine (a) and the contents of whole rock MgO (b) in samples with the sampling elevations

表1 各类橄榄石晶体的成分特征值

Table 1 The composition characteristic values of the populations of olivine crystal

晶体类型	特征值	Fo	w_B/%
晶体类型	特征值	Fo	MnO	CaO	NiO	Cr₂O₃
全部晶体	最大值	92.40	0.57				1.26	0.67	1.09
	最小值	65.33	0.04				0.04	0.04	0.00
	平均值	81.22	0.26				0.26	0.33	0.05
	峰值	83(77)	0.28				0.34	0.36	0.05
粗晶	最大值	90.57	0.57				1.26	0.57	1.09
	最小值	65.33	0.06				0.05	0.06	0.00
	平均值	81.25	0.27				0.25	0.31	0.05
	峰值	84(77)	0.28				0.34	0.37	0.05
聚斑晶	最大值	90.91	0.46				0.51	0.67	0.25
	最小值	69.19	0.07				0.05	0.04	0.00
	平均值	85.36	0.19				0.22	0.41	0.08
	峰值	91(77)	0.16				0.28	0.51	0.1(0.06)
包裹体	最大值	83.30	0.48				0.95	0.47	0.13
	最小值	74.03	0.17				0.04	0.07	0.00
	平均值	80.41	0.27				0.29	0.31	0.03
	峰值	82	0.28				0.34	0.33	0.03
鬣刺晶	最大值	92.40	0.55				0.42	0.51	0.08
	最小值	70.29	0.04				0.16	0.18	0.00
	平均值	76.58	0.33				0.30	0.31	0.03
	峰值	83	0.34				0.34	0.36	0.02

图6 苦橄玢岩中4类橄榄石Fo值频率分布直方图

Fig.6 The histogram of the frequency distribution of four kinds of olivine’s Fo in picrite porphyrite

图7 橄榄石中MnO、CaO、NiO和Cr2O3含量与Fo值的关系批式(实线)和分离(虚线)结晶曲线据文献[21],详见正文。

Fig.7 Variations of MnO, CaO, NiO and Cr2O3 contents with Fo value of the olivines. The batch and fractional curves are from [21]. See text for details.

图8 4类橄榄石斑晶Fo值及电子探针氧化物含量对比 C10—PC02-3粗晶橄榄石;A2—PC06-2微晶橄榄石;I2—PC01-3包裹体橄榄石;S2—PC02-4 鬣刺状橄榄石。

Fig.8 The Fo and comparison of the oxide content of four kinds of olivine

图9 矿山梁子苦橄玢岩中橄榄石相应的熔体图中FeO/MgO为摩尔分数比值。

Fig.9 The corresponding melt of olivine in the Kuangshanliangzi

表2 矿山梁子苦橄玢岩中携带岩浆成分的质量平衡计算结果

Table 2 Results from massbalance recalculation of carrier magma composition for the Kuangshanliangzi porphiritic picrites

样品编号	w_B/%											Mg^#	X^Fe/X^Mg
样品编号	Ol	SiO₂	TiO₂	Al₂O₃	TFeO	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅	Mg^#	X^Fe/X^Mg
PC02-2	31.46	50.26	2.10	14.96	8.65	0.18	6.99	13.57	2.54	0.34	0.18	0.59	0.69
PC02-3	45.89	49.74	1.88	15.07	8.48	0.16	6.85	14.81	2.28	0.33	0.18	0.59	0.69
PC02-4	31.99	49.75	1.83	15.02	9.03	0.18	7.29	13.95	2.19	0.36	0.16	0.59	0.69
PC06-2	11.17	48.46	2.16	14.09	11.01	0.17	8.90	12.00	2.55	0.21	0.19	0.59	0.69
PC06-3	42.96	49.49	1.94	14.60	9.08	0.17	7.34	14.27	2.40	0.34	0.15	0.59	0.69
PC06-4	41.92	50.37	2.07	14.81	9.22	0.15	7.45	13.22	1.76	0.54	0.18	0.59	0.69
PC10-2	11.76	49.51	2.04	14.13	10.26	0.18	8.29	12.23	2.72	0.23	0.14	0.59	0.69
PC10-3	47.17	49.59	1.88	14.96	8.54	0.16	6.90	14.88	2.37	0.32	0.18	0.59	0.69
PC10-4	44.66	50.13	2.05	14.98	9.08	0.16	7.33	13.50	2.13	0.24	0.16	0.59	0.69
PC01-2	34.39	49.89	2.00	13.97	9.80	0.17	7.92	13.29	2.22	0.33	0.17	0.59	0.69
PC01-3	41.66	49.46	1.76	14.54	9.35	0.15	7.56	14.33	1.89	0.54	0.17	0.59	0.69
PC01-4	41.63	49.84	2.14	14.83	9.36	0.17	7.56	13.42	1.98	0.29	0.15	0.59	0.69
PC14-2	39.61	48.93	2.20	14.33	8.86	0.25	7.16	17.23	0.48	0.17	0.17	0.59	0.69
PC14-3	42.35	49.47	2.05	15.00	9.00	0.18	7.27	14.36	1.80	0.45	0.19	0.59	0.69
PC14-4	43.76	50.21	2.11	14.68	8.49	0.16	6.85	16.21	0.57	0.33	0.18	0.59	0.69
PC03	7.89	48.53	1.88	13.31	11.60	0.15	9.37	12.24	2.14	0.37	0.11	0.59	0.69
PC16	29.44	49.71	1.77	13.49	10.74	0.17	8.68	12.37	2.17	0.36	0.27	0.59	0.69
PC35	2.78	49.42	1.74	12.93	11.83	0.19	9.56	11.61	2.10	0.22	0.09	0.59	0.69
PC36	25.33	50.26	1.72	13.15	10.80	0.16	8.72	12.19	2.20	0.27	0.26	0.59	0.69
PC42	27.98	49.70	1.79	13.29	10.82	0.19	8.74	12.76	1.96	0.32	0.16	0.59	0.69

图10 矿山梁子苦橄质岩席在TAS图解(a)和(Na2O+K2O)-MgO图解(b)中的投点位置,示调整前(黑三角)、后(实心圆)的成分差异。(全岩化学分析据[19],底图据[12])

Fig.10 The position of the picrite mat of the Kuangshanliangzi in the TAS diagram (a) and (Na2O+K2O)-MgO diagram (b), the composition difference of the former (solid circle) and after (Solid round) the adjustment. The total rock chemical analysis is according to [19], base figure according to [12].

图11 矿山梁子苦橄玢岩的(FeO/CaO-3×MgO/SiO2)-MgO图解(底图据[30])

Fig.11 The (FeO/CaO-3×MgO/SiO2)-MgO diagram of picrite in the Kuangshanliangzi. Based on [30].

图12 矿山梁子苦橄质岩浆的上升路径(底图据[28]简化) 粗实线为地幔潜温曲线;细实线为干二辉橄榄岩固相线;灰色细实线为温度较低的3种地幔潜温曲线。

Fig.12 The ascending path of the picrite magma of the Kuangshanliangzi. Base figure adapted from [28].

图13 大板山岩体南天湾地区辉长岩中的橄榄石Fo值

Fig.13 The Olivine’s Fo of the south tianwan area in Dabanshan

图14 平川苦橄玢岩的多级岩浆房系统

Fig.14 A multistage magma chamber system of Pingchuan picrite

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