Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire
Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire
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DOI：
10.2138/am-2003-2-311
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发表时间：
2003-02
影响因子：
3.1
通讯作者：
J. Pyle;F. Spear
中科院分区：
文献类型：
--
作者：
J. Pyle;F. Spear
新罕布什尔州Gilsum附近的混合片麻岩中发现的矿物成分和反应结构限制了峰值变质压力（P）和温度（T）条件以及PT路径。大型钾长石变斑晶表明样品在冷却路径上的P4 kb处等压加热。已鉴定出四代独居石，其中三代独居石与特定的全岩反应有关。独居石（4）（最后一代）是在熔体结晶和消耗石榴石和堇青石的过程中与磷钇矿±磷灰石一起产生的。独居石（3）生长在不含磷钇矿的矿物组合中，因为石榴石+白云母反应形成硅线石+黑云母。独居石（2）生长在含磷钇矿的±石榴石+黑云母+石榴石组合中，因为磷钇矿和黑云母在石榴石生产过程中被消耗。独居石（1）与特定的反应无关;它可能是碎屑，记录了早期的变质事件，或代表磷钇矿的不平衡过度生长。YAG独居石、YAG磷钇矿和独居石-磷钇矿温度测定法（2）-（4）产生的温度与主相温度计一致。
Abstract Mineral compositions and reaction textures found in migmatitic gneisses from near Gilsum, New Hampshire, constrain both peak metamorphic pressure (P) and temperature (T) conditions and the PT path. Large K-feldspar porphyroblasts indicate isobaric heating of the samples at P 4 kb on the cooling path. Four generations of monazite have been identified, three of which have been linked to specific whole-rock reactions. Monazite (4) (the last generation) was produced with xenotime ± apatite during melt crystallization and consumption of garnet and cordierite. Monazite (3) grew in a xenotime-absent mineral assemblage as garnet + muscovite reacted to form sillimanite + biotite. Monazite (2) grew in a xenotime-bearing ± garnet + biotite + chlorite assemblage, as xenotime and chlorite were consumed during garnet production. Monazite (1) has not been linked to a specific reaction; it may be detrital, record an earlier metamorphic event, or represent disequilibrium overgrowths of xenotime. YAG-monazite, YAG-xenotime, and monazite-xenotime thermometry for monazite generations (2)-(4) yield temperatures consistent with major-phase thermometers.