引用本文
王成, 任利民, 张晓军, 等. 内蒙古崇根山蛇绿岩中铬铁矿特征及其构造意义[J]. 吉林大学学报(地球科学版), 2019, 49(6): 1552-1564
Wang Cheng, Ren Limin, Zhang Xiaojun, et al. Characteristics and Tectonic Significance of Chromites from Chonggenshan Ophiolite in Inner Mongolia[J]. Journal of Jilin University(Earth Science Edition), 2019, 49(6): 1552-1564.
内蒙古崇根山蛇绿岩中铬铁矿特征及其构造意义
1. 新疆地质调查院, 乌鲁木齐 830000;
2. 中国地质大学(武汉)地球科学学院, 武汉 430074;
3. 中国地质大学(武汉)资源学院, 武汉 430074;
4. 湖北省地质局第一地质大队, 湖北 黄石 435100;
5. 江西省核工业地质局266大队, 南昌 330038
2. 中国地质大学(武汉)地球科学学院, 武汉 430074;
3. 中国地质大学(武汉)资源学院, 武汉 430074;
4. 湖北省地质局第一地质大队, 湖北 黄石 435100;
5. 江西省核工业地质局266大队, 南昌 330038
摘要: 为探讨内蒙古贺根山蛇绿岩形成时的物化条件和大地构造环境，选取崇根山蛇绿岩块中未蚀变副矿物铬铁矿进行矿物学和矿物地球化学分析。结果表明：铬铁矿成分均一，不发育环带结构；铬铁矿中的Cr2O3质量分数为38.86%~39.12%，Al2O3质量分数为26.56%~26.92%，TFeO质量分数为17.37%~17.50%，Cr#为49.34~49.58，Mg#为68.82~69.73，Yfe为6.62~6.97，Fe2+#为30.27~31.18，为富镁铝铬铁矿，其寄主岩石为蛇绿岩；铬铁矿结晶平均温度为1 396.88℃，平均压力为3.35 GPa，推断其形成深度约为103.72 km；相对于FMQ（fayalite magnetite quartz）缓冲剂的地幔氧逸度为FMQ+1.41~FMQ+1.45 lg单位（平均值为FMQ+1.43 lg单位）；地幔熔融程度为20.93%~20.95%（平均值为20.94%）。推测崇根山铬铁矿原岩橄榄岩单元源区为石榴石二辉橄榄岩，形成于亏损地幔。崇根山蛇绿岩为消减带型蛇绿岩，可能产自俯冲带环境中的洋内弧后盆地环境。
关键词:
铬铁矿 蛇绿岩 弧后盆地 崇根山 贺根山
Characteristics and Tectonic Significance of Chromites from Chonggenshan Ophiolite in Inner Mongolia
1. Xinjiang Geological Survey Academy, Urumchi 830000, China;
2. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
4. No. 1 Geological Party of Hubei Geological Bureau, Huangshi 435100, Hubei, China;
5. No. 266 Geological Party of Jiangxi Nuclear Industrial Geological Bureau, Nanchang 330038, China
2. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
3. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
4. No. 1 Geological Party of Hubei Geological Bureau, Huangshi 435100, Hubei, China;
5. No. 266 Geological Party of Jiangxi Nuclear Industrial Geological Bureau, Nanchang 330038, China
Abstract: In order to analyze the formation physicochemical conditions and tectonic environment of the Hegenshan ophiolite in Inner Mongolia, the unaltered chromites from the Chonggenshan ophiolite were chosen to study their mineralogy and mineral geochemistry. The result shows that the chromites consist of Cr2O3 (38.86%-39.12%), Al2O3 (26.56%-26.92%), and TFeO (17.37%-17.50%), Cr# values of 49.34-49.58, Mg#values of 68.82-69.73, Yfe values of 6.62-6.97, and Fe2+#values of 30.27-31.18, without texture zoning. They are aluminum-rich chromites. The average crystallization temperature and pressure of the chromite are estimated to be 1 396.88℃ and 3.35 GPa, respectively, indicating its formation depth of about 103.72 km. It is inferred that the Chonggenshan chromite-hosting peridotite was derived from the garnet lherzolites in depleted upper mantle with oxygen fugacities from FMQ+1.41 to FMQ+1.45 lg units (average FMQ +1.43 lg units), and partial melting degree of the mantle from 20.93% to 20.95% (average 20.94%). The chromites show the characteristics of super-subduction zone. We believe that the Chonggenshan ophiolite was formed in the background of intra-oceanic back-arc basin in the subduction zone.
Key words:
chromite ophiolite intra-oceanic back-arc basin Chonggenshan Hegenshan
0 引言
蛇绿岩标志着板块碰撞的缝合线或者是增生碰撞带, 对于研究古大洋岩石圈的地球化学性质和造山带的演化具有重要作用[1], 主要分为MORB(mid-ocean ridge，大洋中脊)型和SSZ (supra-subduction zone，消减带)型[2-4]。在内蒙古中北部由北到南发育着4条蛇绿岩带, 分别为二连浩特—贺根山蛇绿岩带、交其尔—锡林浩特蛇绿岩带、索伦敖包—林西蛇绿岩带和温都尔庙—西拉木伦河蛇绿岩带[5-6], 贺根山蛇绿岩位于最北部的二连浩特—贺根山蛇绿岩带上[6-9]。贺根山蛇绿岩作为构造岩浆作用过程的记录, 能为地幔作用研究、岩石圈增生与裂解过程探讨、板块碰撞和构造环境恢复提供大量有用信息，对于解决内蒙古北部地质构造和演化历史等重大基础地质问题具有重要意义，并已获得广大学者的广泛关注[6-13]。目前贺根山蛇绿岩形成环境还没有达成一致共识, 存在着大洋中脊成因[10-11]和消减带成因[6, 8, 13-24]的争论, 还有少数学者认为可能是超镁铁质成因[12], 但多数学者认为产生于俯冲消减带环境。
铬铁矿相对于其他岩浆成因矿物更加稳定, 其特征参数Cr# 、Mg#、Fe3+#和Fe2+#的变化范围, Al2O3、Cr2O3、MgO、Fe2O3和TiO2等主要氧化物的含量、变化与铬铁矿形成时所处构造环境密切相关, 可以为寄主岩石成因、地幔特征及铬铁矿形成时的大地构造环境的判断提供可靠依据[25-29], 对于蛇绿岩类型的判别至关重要[26, 30]。
贺根山蛇绿岩由贺根山、河北农场、吉斯布敦、哈日海陶勒盖和崇根山等5大蛇绿岩块组成, 其共同组成了由地幔橄榄岩、镁铁质堆晶岩、侵入杂岩、气孔杏仁状玄武岩和放射虫硅质岩构成的完整蛇绿岩套。目前，前人的研究主要针对发育有豆荚状铬铁矿的贺根山蛇绿岩块, 而对崇根山蛇绿岩块研究相对较少, 且主要集中于岩石学、岩石地球化学和同位素定年方面[14-15, 21, 31-32], 对于橄榄岩中赋存的副矿物铬铁矿研究尚属空白。本文主要从铬铁矿矿物地球化学特征的角度探讨寄主橄榄岩形成的构造环境, 以期为二连浩特—贺根山缝合带的构造演化提供地球化学证据。
1 地质背景崇根山蛇绿岩块位于二连浩特—贺根山缝合带中段(图 1), 为贺根山蛇绿岩中出露面积最大(超过300 km2)、岩性组合最为齐全的蛇绿岩块(图 2)。崇根山蛇绿岩块北端出露的地层有石炭系—二叠系格根敖包组(C-Pg)[34-35]海相火山岩和中二叠统哲斯组(P2z)[36-38]滨海—浅海相正常沉积碎屑岩, 蛇绿岩块与哲斯组为断层接触, 被格根敖包组喷发不整合覆盖; 南端出露中—上泥盆统塔尔巴格特组(D2-3t)[39]海相浅变质程度的砂板岩, 其角度不整合覆盖于蛇绿岩块之上; 其余大部分地段被新生代上新统宝格达乌拉组(N2b)陆相松散堆积物或第四系(Q)残坡积物与冲洪积物覆盖。崇根山蛇绿岩块主要由地幔橄榄岩、少量堆晶辉长-辉绿质岩墙和拉斑玄武岩片组成, 相互之间呈断层接触。地幔橄榄岩主要为方辉橄榄岩、纯橄榄岩和二辉橄榄岩, 岩石蛇纹石化蚀变十分强烈且普遍, 相互之间为断层接触(图 2)。其岩块北部以强蛇纹石化方辉橄榄岩和纯橄榄岩为主, 而南部则除了前述的岩片外, 还有大量的二辉橄榄岩。堆晶辉长-辉绿质岩墙出露虽然零星, 且叠加了后期绿帘石—角闪岩相的变质作用, 但其堆晶特征清晰, 是构成本区蛇绿岩套的重要组成部分[13, 33](图 3)。崇根山蛇绿岩块中副矿物铬铁矿较为常见, 但至今未见到有豆荚状铬铁矿的报道。本次研究的副矿物铬铁矿主要发育于崇根山强蛇纹石化方辉橄榄岩中。
方辉橄榄岩为崇根山地幔橄榄岩的主体, 出露面积大且连续, 约占蛇绿岩块出露总面积的50%, 抗风化能力较强, 宏观为山丘(图 4a)。方辉橄榄岩灰绿—暗绿色, 具粒状结构, 块状构造, 由橄榄石(80%~85%)、斜方辉石(10%~15%)和单斜辉石(2%)组成(图 4b)。橄榄石半自形粒状, 大小1.0~2.5 mm, 多已发生强蛇纹石化, 形成特征的网格构造, 裂隙面发育纤蛇纹石, 表面发育薄层利蛇纹石；斜方辉石半自形粒状, 大小1.0~6.0 mm, 填隙状分布在橄榄石之间, 多发生滑石化、闪石化和蛇纹石化；单斜辉石较少, 他形粒状, 大小0.5~2.0 mm, 填隙状分布。副矿物为铬铁矿(2%), 粒径0.05~1.50 mm, 他形, 多为近似椭圆状, 环带不发育, 分布于蛇纹石(化矿物)间, 正交偏光镜下全消光, 在单偏光镜下颗粒中心部位到边缘部位均为暗灰色, 未见明显的颜色差异, 铬铁矿中裂隙不发育。
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| a.片理化方辉橄榄岩; b.方辉橄榄岩(单偏光); c，d.铬铁矿(背散射)。Ol.橄榄石; Opx.斜方辉石; Sep.蛇纹石; Chr.铬铁矿。 图 4 研究区野外及显微照片 Fig. 4 Photographs of ophiolite outcrops and microphotographs of the Chonggenshan ophiolite |
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崇根山副矿物铬铁矿基本无蚀变, 成分较均一, 未见明显的环带特征。本文选取新鲜无蚀变铬铁矿进行了电子探针分析, 测试工作在中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成, 实验仪器型号为JXA-8 100, 分析条件为加速电压15 kV, 束流100 nA, 电子束径1 μm, 工作精度达100×10-6。Fe价态对于下文铝铬铁矿温度、压力和氧逸度等物化条件的计算至关重要, 需将电子探针数据中TFeO折算成FeO和Fe2O3, 本文利用化学式(AB2O4)和电价平衡计算出Fe2+和Fe3+摩尔分数, 再以4个氧原子为基础, 计算求得全部样品的阳离子数[17, 29]。分析和计算结果见表 1。
表 1 崇根山铬铁矿测试数据
Table 1 Analytical results of podiform chromites from the Chonggenshan ophiolite
| 点位号 | TFeO | TiO2 | SiO2 | MgO | Al2O3 | Cr2O3 | MnO | CoO | NiO | V2O3 | Na2O | K2O | CaO | 总和 |
| CG-1-1 | 17.50 | 0.42 | 0.02 | 15.36 | 26.92 | 39.12 | 0.24 | 0.00 | 0.10 | 0.20 | 0.03 | 0.00 | 0.00 | 99.90 |
| CG-1-2 | 17.47 | 0.42 | 0.02 | 15.27 | 26.78 | 39.10 | 0.24 | 0.00 | 0.11 | 0.21 | 0.02 | 0.00 | 0.00 | 99.63 |
| CG-1-3 | 17.44 | 0.43 | 0.02 | 15.09 | 26.73 | 38.90 | 0.24 | 0.00 | 0.11 | 0.22 | 0.01 | 0.01 | 0.01 | 99.17 |
| CG-1-4 | 17.44 | 0.41 | 0.02 | 15.22 | 26.81 | 38.96 | 0.24 | 0.00 | 0.11 | 0.21 | 0.02 | 0.01 | 0.00 | 99.43 |
| CG-2-1 | 17.49 | 0.42 | 0.01 | 15.16 | 26.68 | 39.10 | 0.24 | 0.00 | 0.12 | 0.21 | 0.01 | 0.00 | 0.00 | 99.45 |
| CG-2-2 | 17.49 | 0.42 | 0.02 | 15.19 | 26.74 | 38.99 | 0.24 | 0.00 | 0.11 | 0.21 | 0.01 | 0.00 | 0.00 | 99.43 |
| CG-2-3 | 17.38 | 0.43 | 0.02 | 15.21 | 26.79 | 38.88 | 0.23 | 0.00 | 0.11 | 0.20 | 0.02 | 0.01 | 0.01 | 99.27 |
| CG-3-1 | 17.43 | 0.42 | 0.02 | 15.18 | 26.77 | 38.99 | 0.24 | 0.00 | 0.11 | 0.21 | 0.01 | 0.00 | 0.00 | 99.39 |
| CG-3-2 | 17.43 | 0.41 | 0.02 | 15.28 | 26.75 | 39.08 | 0.24 | 0.00 | 0.11 | 0.20 | 0.02 | 0.00 | 0.00 | 99.53 |
| CG-3-3 | 17.39 | 0.41 | 0.02 | 15.16 | 26.70 | 38.94 | 0.23 | 0.00 | 0.11 | 0.21 | 0.01 | 0.01 | 0.01 | 99.19 |
| CG-4-1 | 17.37 | 0.42 | 0.01 | 15.01 | 26.56 | 38.86 | 0.23 | 0.00 | 0.12 | 0.21 | 0.00 | 0.01 | 0.01 | 98.80 |
崇根山铬铁矿Cr2O3质量分数为38.86%~39.12%, 平均值为38.99%;A12O3质量分数为26.56%~26.92%, 平均值为26.75%;TFeO质量分数为17.37%~17.50%, 平均值为17.44%(表 1); 崇根山铬铁矿Cr#为49.34~49.58, 平均值为49.45;Mg#为68.82~69.73, 平均值为69.31;Yfe为6.62~6.97, 平均值为6.81;Fe2+#为30.27~31.18, 平均值为30.69(表 2)。在铬铁矿分类图(图 5)中显示为富镁铝铬铁矿, 与贺根山蛇绿岩块豆荚状铬铁矿主体为铝铬铁矿、少数为富铬尖晶石和铬铁矿的情况明显不同[17]。
表 2 崇根山铬铁矿计算参数
Table 2 Related parameters of podiform chromites from the Chonggenshan ophiolite
| 点位号 | Fe2+ | Fe3+ | Ti | Si | Mg | Al | Cr | Mn | Co | Ni | V | Na | K | Ca | Cr# | Mg# | Yfe | Fe2+# | p/GPa | H/km | t/℃ | Δlg(fo2)FMQ | F/% |
| CG-1-1 | 0.589 | 0.417 | 0.037 | 0.003 | 1.356 | 2.817 | 2.747 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.37 | 69.73 | 6.97 | 30.27 | 3.35 | 103.73 | 1 397.20 | 1.45 | 20.93 |
| CG-1-2 | 0.594 | 0.412 | 0.037 | 0.002 | 1.353 | 2.813 | 2.756 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.49 | 69.49 | 6.89 | 30.51 | 3.35 | 103.78 | 1 396.90 | 1.44 | 20.94 |
| CG-1-3 | 0.605 | 0.400 | 0.038 | 0.002 | 1.344 | 2.822 | 2.756 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.41 | 68.94 | 6.68 | 31.06 | 3.34 | 103.63 | 1 396.80 | 1.41 | 20.94 |
| CG-1-4 | 0.594 | 0.412 | 0.037 | 0.000 | 1.351 | 2.821 | 2.752 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.37 | 69.46 | 6.88 | 30.54 | 3.34 | 103.69 | 1 397.10 | 1.44 | 20.93 |
| CG-2-1 | 0.602 | 0.405 | 0.038 | 0.002 | 1.347 | 2.81 | 2.763 | 0.012 | 0.000 | 0.006 | 0.015 | 0.001 | 0.000 | 0.000 | 49.58 | 69.12 | 6.78 | 30.88 | 3.35 | 103.81 | 1 396.60 | 1.43 | 20.95 |
| CG-2-2 | 0.598 | 0.411 | 0.037 | 0.002 | 1.349 | 2.815 | 2.754 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.45 | 69.29 | 6.86 | 30.71 | 3.35 | 103.74 | 1 396.90 | 1.44 | 20.94 |
| CG-2-3 | 0.596 | 0.406 | 0.039 | 0.003 | 1.352 | 2.823 | 2.749 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.34 | 69.40 | 6.79 | 30.60 | 3.34 | 103.62 | 1 397.10 | 1.43 | 20.93 |
| CG-3-1 | 0.599 | 0.405 | 0.037 | 0.002 | 1.349 | 2.819 | 2.755 | 0.012 | 0.000 | 0.005 | 0.015 | 0.001 | 0.000 | 0.000 | 49.43 | 69.24 | 6.77 | 30.76 | 3.34 | 103.68 | 1 396.90 | 1.43 | 20.94 |
| CG-3-2 | 0.592 | 0.414 | 0.037 | 0.002 | 1.355 | 2.811 | 2.756 | 0.012 | 0.000 | 0.006 | 0.015 | 0.001 | 0.000 | 0.000 | 49.51 | 69.62 | 6.91 | 30.38 | 3.35 | 103.81 | 1 396.90 | 1.44 | 20.95 |
| CG-3-3 | 0.598 | 0.406 | 0.037 | 0.002 | 1.35 | 2.817 | 2.757 | 0.012 | 0.000 | 0.006 | 0.015 | 0.001 | 0.000 | 0.000 | 49.46 | 69.29 | 6.78 | 30.71 | 3.35 | 103.72 | 1 396.80 | 1.43 | 20.94 |
| CG-4-1 | 0.608 | 0.396 | 0.038 | 0.001 | 1.342 | 2.816 | 2.765 | 0.012 | 0.000 | 0.006 | 0.015 | 0.000 | 0.000 | 0.001 | 49.55 | 68.82 | 6.62 | 31.18 | 3.35 | 103.71 | 1 396.50 | 1.41 | 20.95 |
| 注：Cr#=100 Cr/(Cr+Al)；Mg#=100 Mg/(Mg+Fe2+)；Yfe=100 Fe3+/(Fe3++Al+Cr)；Fe2+#=100 Fe2+/(Fe2++Mg)，其中Cr、Al、Mg、Fe2+、Fe3+为阳离子数目。平均值：Cr#为49.45,Mg#为69.31,Yfe为6.81,Fe2+#为30.69, p为3.35 GPa,H为103.72 km，t为1 396.88 ℃,Δlg(fo2)FMQ为FMQ+1.43 lg单位，F为20.94%。 | |||||||||||||||||||||||
铬铁矿成分环带产生的原因有两种, 一种是岩浆结晶分异[41], 另一种是超基性岩浆成因, 即超基性岩经受变质作用、不同物质组成对应不同变质相的结果[42]。原生铬铁矿也可以不发育环带, 随着铬铁矿自形程度的降低和粒度变细, 成分环带逐渐消失[43]。原生铬铁矿受到后期蚀变改造后, 核部到边部Fe的质量分数会逐渐升高, Cr和Mg的质量分数会变化, 蚀变越强，Fe质量分数越高, 当完全蚀变后, 铬铁矿会变成褐铁矿和磁铁矿。一般而言, 后期遭受蚀变的铬铁矿通常会发育成分分带, 除非铬铁矿毫无残留全部蚀变(褐铁矿和磁铁矿)。本文所测试铬铁矿为高镁铬铁矿, 铁质量分数较低, 未见明显环带特征, 成分较为均一, 颗粒间无明显变化, 未发生明显蚀变, 表明各颗粒处于平衡状态[44], 同时裂隙也不发育, 表明副矿物铬铁矿在崇根山蛇绿岩后期俯冲就位的过程中受到的改造较少, 可以代表原始的铬铁矿，并且可以用于本文后续的参数计算和相关论证分析。
在Cr#-Mg#构造环境判别图解(图 6)中, 崇根山铬铁矿主要位于俯冲带(SSZ)型蛇绿岩、阿尔卑斯型地幔橄榄岩和深海橄榄岩的重叠部位。本文所测结果(Cr#=49.34~49.58和Mg#=68.82~69.73), 显示出该区橄榄岩既不同于典型的大洋上地幔物质, 也与标准的大陆上地幔成因有区别, 与纽芬兰岛湾蛇绿岩和阿曼赛迈尔蛇绿岩相似[26, 45]。根据以上判断结果，认为铬铁矿的寄主原岩方辉橄榄岩应属于蛇绿岩成员, 这和区域上前人研究成果一致[6, 8, 10-11, 13-24]。
根据O’Neill[47]提出的铬铁矿压力计算公式, 计算得到崇根山铬铁矿结晶压力为3.34~3.35 GPa, 其平均值为3.35 GPa, 推测其形成深度H(31p)约为103.72 km。
根据Fabriès[48]提出的温度计算公式，得出崇根山铬铁矿结晶温度t(T-273.15)平均值为1 396.88 ℃。软流圈温度为1 280~1 350 ℃[49], 与本文通过铬铁矿计算出来的结果很接近，由此推测铬铁矿寄主岩石形成于地幔环境, 处于石榴石二辉橄榄岩的稳定相区内(图 7), 即铬铁矿寄主橄榄岩方辉橄榄岩原岩为石榴石二辉橄榄岩。
联合采用Irvine[41]、Fabriès[48]和Fudali[52]提出的氧逸度公式，计算得出崇根山铬铁矿相对氧逸度Δlg(fo2)FMQ为FMQ +1.41~FMQ+1.45 lg单位, 平均值为FMQ+1.43 lg单位。
选用Hellebrand等[53]提出的部分熔融公式进行计算, 估算崇根山铬铁矿部分熔融程度F为20.93%~20.95%, 平均值为20.94%(表 2)。
4.3 构造环境讨论在Cr#-Mg#形成环境判别图(图 6)中, 铬铁矿位于SSZ蛇绿岩区域(图 6b), 又处于深海橄榄岩范围内(图 6a)。在判别地幔亏损程度的Cr#-w(TiO2)关系图(图 8a)中, 可以看出铬铁矿落入亏损地幔橄榄岩与洋中脊玄武岩过渡区域, 更加偏向于洋中脊玄武岩区。在w(TiO2)-w(Al2O3)构造环境判别关系图(图 8b)中, 铬铁矿主要落入俯冲带橄榄岩(SSZP)、大洋中脊橄榄岩(MORP)和现代弧后盆地中尖晶石(BAB)中的重叠区域。在Fe2+/Fe3+-w(Al2O3)图解(图 9)中, 铬铁矿落入洋中脊型橄榄岩区域。同时, 铬铁矿的Fe3+/ Fe2+范围为(0.65~0.71)＞0.5, 与贺根山豆荚状铬铁矿区别较大[17]; TiO2和Al2O3质量分数负相关关系不明显, 特征明显不同于来自典型MORB的铬尖晶石[56]。
王成等[17]对贺根山蛇绿岩块中的豆荚状铬铁矿进行了研究，其选矿铬尖晶石的各项参数(结晶温度与压力、形成深度、氧逸度和地幔熔融程度等)与本文获得的结果十分接近，可进行对比，说明二者形成于相同的构造环境。结合区域构造背景，同时加强其他区域典型蛇绿岩的对比[57], 认为崇根山蛇绿岩为SSZ型, 形成于俯冲带环境中洋内弧后盆地环境。
5 结论1) 崇根山铬铁矿主要以副矿物的形式存在, 蚀变程度弱, 为高镁铝铬铁矿。
2) 崇根山铬铁矿的结晶温度为1 396.50~1 397.20 ℃, 平均值为1 396.88 ℃; 结晶压力为3.34~3.35 GPa, 平均值为3.35 GPa, 推测其形成深度为103.72 km; 所处的地幔氧逸度为FMQ +1.41~FMQ+1.45 lg单位, 平均值为FMQ+1.43 lg单位; F为20.93%~20.95%, 平均值为20.94%。
3) 崇根山铬铁矿的寄主岩石方辉橄榄岩属于蛇绿岩成员, 原岩为石榴石二辉橄榄岩。
4) 崇根山蛇绿岩为SSZ型蛇绿岩, 形成于俯冲带环境中洋内弧后盆地环境。
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