桐柏山造山带南麓橄长质岩墙（床）群的矿物化学、地球化学特征与成因意义

桐柏山造山带南麓随州枣阳地区的超镁铁镁铁质岩墙(床)群分为性质完全不同的两类:橄长质岩墙(床)群和变质的基性岩墙群。其中橄长质岩墙(床)群未经历过任何变形和变质作用的影响,其侵位时代应不早于区内高压超高压变质作用的时间(220~240Ma),可能属晚造山或后造山岩浆作用的产物。这些橄长质岩墙(床)群中主要造岩矿物为橄榄石、斜长石、斜方辉石和单斜辉石,含少量金云母。其中橄榄石的Fo介于72~78之间,为贵橄榄石;斜长石以倍长石为主,少数为拉长石;单斜辉石主要为普通辉石,少数为透辉石;金云母中含较高的Ti O2(3.36%),属钛金云母变种。橄榄石和普通辉石之间的平衡温度主要介于1120~1165℃之间,普通辉石的结晶温度为971~1079℃。橄长质岩墙(床)群在地球化学上以高Al2O3(17.25%~20.20%)和MgO(9.56%~15.30%)含量、高的MgO/FeO*值(1.26~1.76)以及低Ti O2(0.33%~0.58%)和低碱(Na2O K2O=1.64%~2.23%)为特征,过渡元素的丰度(Cr=70~125μg/g、Ni=251~518μg/g、Co=54~75μg/g)和Mg#值(54~67)均较高,其性质可近似代表上地幔部分熔融所形成的母岩浆。稀土元素含量低(ΣREE=18.66~35.42μg/g),轻、重稀土元素之间的分馏程度较强,(La/Yb)N=4.4~4.7,尤以强的Ba、Sr及Eu正异常(δEu=1.31~1.49)为显著特征,表明其中有较多的富钙斜长石聚集。高场强元素Nb、Ta、U、Th以及Zr和Hf无明显异常,不同于区内早古生代玄武岩所揭示的富集型上地幔的地球化学特征,而与造山带内及前、后陆地区古近纪玄武岩的地球化学特征非常相似,可能表明造山带山根拆沉后,发生过软流圈地幔与大陆岩石圈地幔强烈的相互作用。橄长质岩墙群的出现说明俯冲板块的后缘曾发生过岩石圈范围的强烈拉张作用。     

Petrogenesis of sapphirine-bearing metatroctolites from the Buck Creek ultramafic body,southern Appalachians

The Buck Creek ultramafic body, North Carolina, includes aluminous lenses that have been described as troctolites. These lenses preserve mineral assemblages which record several different stages of metamorphism. The first stage is characterized by anhydrous reactions between olivine and plagioclase to produce coronas of orthopyroxene+ clinopyroxene/spinel symplectite. Thermo barometric results indicate minimum pressures of c. 6 kbar and c. 800 ^oC. Sapphirine replaces spinel in some clinopyroxene symplectites, and occurs as anhedral grains within amphibole, observations which in combination suggest peak metamorphic conditions of c. 9-10 kbar and c. 850 ^oC. Sapphirine-bearing hydrous assemblages formed at the expense of the coronas, indicating a second metamorphic episode involving deeper burial, deformation and hydration. Schistose rocks from the margins of the lenses are composed of anorthite+amphibole+margarite+corundum, and probably record a later, lower P-T event. Whole rock analyses for the Buck Creek lenses suggest an accumulate protolith of magnesian olivine and calcic plagioclase. Trace element data for the troctolites are consistent with data for adjacent amphibolites in suggesting that the Buck Creek mafic and ultramafic cumulates crystallized from magmas derived from a mantle source similar to that which produces modern intraplate or rift-related basalts. We propose that the Buck Creek ultramafics represent basal cumulates(± uppermost mantle) from ocean crust formed in a marginal basin in the latest Precambrian. Subduction-induced burial to at least 18 km under dry conditions induced corona formation. Collisional events of the Taconic orogeny thrust the Buck Creek rocks into the orogenic pile to at least 30 km depth and hydrated them along zones of weakness, locally producing P-T -P_H2O conditions appropriate for formation of sapphirine and hydrated assemblages, but still preserving some dry symplectites.     

Petrochemical evidence for off-ridge magmatism in a back-arc setting from the Yakuno ophiolite, Japan

Abstract The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)‐ and transitional (T)‐types ([La/Yb]_N = 0.77–11.2) of mid‐oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40–1.20) are as low as those of back‐arc basin basalts (BABB). Cr‐spinels in the metavolcanic rocks have Cr? of 40–73 and an Fe³⁺? of 9–24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back‐arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5‐km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine–plagioclase crystallization sequence, different from the commonly observed olivine–clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb]_N = 0.18–0.55) so that it has an REE pattern that mimics normal (N)‐type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO₂ contents (0.2–1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off‐ridge environment during sea floor spreading of the back‐arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N‐ to E‐MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back‐arc basin.