Mineral paragenesis in Franciscan metagraywackes of the Nacimiento Block, a subduction complex of the Southern California Coast Ranges
Mineral paragenesis in Franciscan metagraywackes of the Nacimiento Block, a subduction complex of the Southern California Coast Ranges
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纳西门托地块（南加州海岸山脉的俯冲复合体）方济各会杂灰砂岩中的矿物共生
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发表时间：
1980
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通讯作者：
W. Ernst
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作者：
W. Ernst
对南加州海岸山脉 Nacimiento 地块的方济会变质灰砂岩进行的地质岩相勘察表明，变质程度明显向东增加。大多数靠近太平洋的弱重结晶区 I 变碎岩富含方解石脉和斑块，并且不含新的钙铝水合硅酸盐；丰富的不稳定碎屑包括岩屑、黑云母片和绿帘石颗粒。内侧 II 区的变沉积岩的特征是细粒共生的绿帘石±5 少量再生绿帘石-斜帘石。最东端的 III 区变砂岩含有硬钠石，很少含有霰石和/或硬玉质辉石。总体而言，向东方解石、碎石、绿帘石、黑云母比例减少，白云母、绿泥石、硅黑云母比例增加。根据实验相平衡数据，估计150±50°C的温度发生了再结晶，岩石静压向东增加，从西部的约2-3kbar增加到约5kbar，东部局部接近8kbar。最低品位 I 区变质粗砂岩中不存在沸石可能表明在变质过程中水相被 CO2 稀释。结合 Nacimiento 地块的西北构造“颗粒”及其推断的叠瓦结构、岩石类型的组合、层理的总体东北倾角以及该剖面主要的正面朝上性质，该大陆边缘矿床被认为是在相对快速（可能是短暂）的晚白垩世板块聚合期间发生再结晶的。与位于圣安德烈亚斯断层东北部的方济各基底相比，缺乏榴辉岩和角闪岩作为构造块可能表明恢复的俯冲深度较浅。变质的纳西门托岩屑带显然是在美国岩石圈板块西缘的一段短暂的俯冲过程中聚集而成的，似乎反映了加利福尼亚州北部海岸山脉的方济会岩石在较长时间间隔内发生的事件。
Geologic-petrographic reconnaissance of Franciscan metagraywackes from the Nacimiento block of the southern California Coast Ranges has demonstrated a perceptible eastward increase in metamorphic grade. Most feebly recrystallized zone I metaclastic rocks bordering the Pacific Ocean are rich in calcite veins and patches and contain no new Ca-Al hydrous silicates; abundant unstable detritus includes lithic fragments, biotite flakes, and epidote granules. Metasedimentary rocks of medial zone II are characterized by fine-grained intergrowths of pumpellyite ±5 minor regenerated epidote-clinozoisite. Easternmost zone III metasandstones contain lawsonite, rarely aragonite and/or jadeitic pyroxene. In general, proportions of calcite, rock fragments, epidote, and biotite decrease eastward, and white mica, chlorite, and stilpnomelane increase. On the basis of experimental phase equilibrium data, temperatures of 150±50°C are estimated to have attended recrystallization, with lithostatic pressures increasing eastward from about 2–3 kbar on the west to approximately 5 kbar, and locally approaching 8 kbar, on the east. Absence of zeolites in the lowest grade zone I metagraywackes may indicate dilution of the aqueous fluid phase by CO2 during the metamorphism. In combination with the northwest tectonic ‘grain’ of the Nacimiento block and its inferred imbricate structure, the assemblage of rock types, the general north-eastward dip of bedding, and predominantly right-side-up nature of the section, this continental margin deposit is thought to have been recrystallized during a period of relatively rapid (and perhaps brief) Late Cretaceous plate convergence. The lack of eclogite and amphibolite as tectonic blocks may indicate less profound depths of recovered subduction compared to Franciscan basement lying northeast of the San Andreas fault. Metamorphosed Nacimiento lithic belts evidently were assembled over a brief period of subduction offscraping at the western edge of the American lithospheric plate and seem to mirror events which unfolded over a longer time interval in Franciscan rocks of the northern California Coast Ranges.