Mineral chemistry of a Cenozoic igneous complex,the Urumieh–Dokhtar magmatic belt,Iran: Petrological implications for the plutonic rocks

The Niyasar plutonic complex, one of the Cenozoic magmatic assemblages in the Urumieh‐Dokhtar magmatic belt, was the subject of detailed petrographic and mineralogical investigations. The Niyasar magmatic complex is composed of Eocene to Oligocene mafic rocks and Miocene granitoids. Eleven samples, representing the major rock units in the Niyasar magmatic complex and contact aureole were chosen for mineral chemical studies and for estimation of the pressure, temperature, and oxygen fugacity conditions of mineral crystallization during emplacement of various magmatic bodies. The analyzed samples are composed of varying proportions of quartz, plagioclase, K‐feldspar, hornblende, biotite, titanite, magnetite, apatite, zircon, garnet, and clinopyroxene. Application of the Al‐in‐hornblende barometer indicates pressures of around 0.2 to 0.4 kbar for the Eocene–Oligocene mafic bodies and around 0.5 to 1.7 kbar for the Miocene granitoids. Hornblende‐plagioclase thermometry yields relatively low temperatures (661–780 °C), which probably reflect late stage re‐equilibration of these minerals. The assemblage titanite–magnetite–quartz as well as hornblende composition were used to constrain the oxygen fugacity and H₂O content during the crystallization of the parent magmas in the Miocene plutons. The results show that the Miocene granitoids crystallized from magmas with relatively high oxygen fugacity and high H₂O content (~5 wt% H₂O). The Miocene granitoids show similar range of oxygen fugacity, H₂O contents and mineral chemical compositions, which indicate a common source for their magmas. Although the crystallization pressures of the Miocene plutons discriminate various categories of plutonic bodies emplaced at depths of about 5.7–6.5 km (Marfioun pluton), about 4.2 km (Ghalhar pluton) and 1.9–2.3 km (Poudalg pluton), they were later uplifted to the same level by vertical displacement of faults. The emplacement depths of the Niyasar plutons suggest that the central part of the Urumieh‐Dokhtar magmatic belt has experienced an uplift rate of ca. 0.25–0.4 mm/yr from the Miocene onwards.     

Contributions of slab fluid and sediment melt components to magmatism in the Mariana Arc–Trough system: Evidence from geochemical compositions and Sr,Nd, and noble gas isotope systematics

This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, ⁸⁴Kr/⁴He and ¹³²Xe/⁴He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of ³He/⁴He and ⁴⁰Ar/³⁶Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in ²⁰Ne/²²Ne and ²¹Ne/²²Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess ¹²⁹Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.     

Geochemical characteristics of deposits from the 2011 Tohoku‐oki tsunami at Hasunuma,Kujukuri coastal plain,Japan

We examined the geochemical characteristics and temporal changes of deposits associated with the 2011 Tohoku‐oki tsunami. Stable carbon isotope ratios, biomarkers, and water‐leachable ions were measured in a sandy tsunami deposit and associated soils sampled at Hasunuma, Kujukuri coastal plain, Japan, in 2011 and 2014. At this site, the 2011 tsunami formed a 10–30 cm ‐thick layer of very fine to medium sand. The tsunami deposit was organic‐poor, and no samples contained any detectable biomarkers of either terrigenous or marine origin. In the underlying soil, we identified hydrocarbons and sterols derived from terrestrial plants, but detected no biomarkers of marine origin. In the samples collected in 2011, concentrations of tsunami‐derived water‐leachable ions were highest in the soil immediately beneath the tsunami deposit and then decreased gradually with depth. Because of its finer texture and higher organic content, the soil has a higher water‐holding capacity than the sandy tsunami deposit. This distribution suggests that ions derived from the tsunami quickly penetrated the sand layer and became concentrated in the underlying soil. In the samples collected in 2014, concentrations of water‐leachable ions were very low in both soil and sand. We attribute the decrease in ion concentrations to post‐tsunami rainfall, seepage, and seasonal changes in groundwater level. Although water‐leachable ions derived from seawater were concentrated in the soil beneath the tsunami deposit following the tsunami inundation, they were not retained for more than a few years. To elucidate the behavior of geochemical characteristics associated with tsunamis, further research on organic‐rich muddy deposits (muddy tsunami deposits and soils beneath sandy tsunami deposits) as well as sandy tsunami deposits is required.     

2014年5月云南盈江两次中强地震震源参数研究

本文采用“剪切-粘贴”(CAP, Cut And Paste)方法, 分别利用区域地震台网的宽频带记录和全球地震台网宽频带记录, 独立反演和联合反演了云南盈江2014年5月24日5.6级和5月30日6.1级地震震源参数。 结果表明, 采用近震和远震数据联合反演结果更为稳定, 5月24日5.6级地震震源机制解两个节面分别为: 走向153°/倾向90°/滑动角171°和243°/81°/0°, 5月30日6.1级地震两个节面分别为172°/72°/180°和82°/90°/-18°。 两次地震断层参数较为接近, 均为走滑型地震, 结合区域构造背景和地震序列重定位结果认为其发震断层可能为近南北向苏典断裂。 震源机制解反演结果与震源区历史地震震源机制解和利用其它方法得到的机制解相近, 也符合震源区的区域应力场特征。 两次地震反演最佳震源深度分别为8 km和6 km, 均为发生在上地壳的浅源地震, 表明苏典断裂在此部位可能并没有切割到地壳深部。     

重复地震识别的参数选取综述

分析滤波频段、台站密度及互相关系数阈值等参数的选取与重复地震识别率的关系,对“波形相关”意义上的重复地震与物理意义上震源区的重复地震之间的关系进行定性讨论。结果表明,尽可能增加可用台站数、多使用近震资料、选取约0.2～20 Hz的滤波频段及不小于0.8的互相关系数阈值,可以较好地识别重复地震。此外,当重复地震事件MS ≥ 4.5时,两个事件的震源区至少有部分重合。     

兴都库什–帕米尔地区与天山地震带强震活动关联分析*

首先通过对区域地质构造背景、地震活动性质以及动力环境的分析,认为天山地震带强震活动主要受兴都库什-帕米尔构造结的动力控制。其次分析了兴都库什-帕米尔地区与天山地震带强震活动之间的相关关系,结果表明两地区强震成组活动存在一定的同步特征;进一步考察兴都库什-帕米尔地区中源地震与天山地震带强震活动之间关系,发现两者同样存在同步特征,且该区中源地震的活动强度和频度越大,天山地震带的强震活动越剧烈,其分布范围也越广。该结果从地震活动的角度反映了兴都库什-帕米尔地区对天山地震带强震活动的动力控制作用。     

1988年澜沧—耿马地震前震源区应力状态分析

利用1973年1月1日至1988年11月5日云南区域地震台网记录的地震资料,获得了1988年澜沧—耿马地震前震源及附近区域多个地震活动性参数的空间分布图像,初步揭示了震前震源区应力状态.结果表明:澜沧—耿马地震之前,研究区地震活动性参数分布具有显著的空间不均匀性特征.澜沧7.6级地震震源区以低于0.7的异常低b值、低a值以及整个研究区最短的局部复发间隔Tl值为特征,表明震源所在断裂段在震前已处于高应力闭锁状态,具有发生大震的应力条件.     

使用中国地震台网资料快速测定中强地震矩震级

利用中国地震台网数字地震资料,选取国内Ms≥5.3地震波形数据,采用长周期波形拟合方法,快速计算2008年至今86个中强震的标量地震矩MO和矩震级Mw,并将Mw与GCMT的测定结果及中国地震台网中心测定的面波震级Ms进行对比.结果表明：矩震级与GCMT测定一致,与中国地震台网中心测定的面波震级关系为Mw=0.86Ms＋0.63.     

双差法在江苏及邻域地震事件重新定位中的应用

通过双差定位法,对江苏及邻域1990-2012年地震事件进行重新定位,其中包括部分人工爆破和塌陷事件。结果显示,重新定位后的爆破和塌陷事件震源深度均有明显改善;地震在空间分布上更加集中,部分地震有向构造带趋近的变化,较多地震呈丛集状出现,震源深度主要集中在5-25 km范围内。     

毛乌素沙地风沙沉积物磁学特征及其古环境意义

在沙漠沉积环境中,成土作用对磁化率的贡献较小,往往被原生磁信号掩盖,因此分离两种磁组分对气候和粉尘代用指标的提取至关重要.本研究选取位于毛乌素沙地东缘的锦界风沙沉积剖面为研究对象,利用多变量一元线性回归中的"平均值概念"进行磁化率的原生碎屑组分0和次生成土组分Xpedo的分离,并探讨磁学参数所承载的气候和环境意义.结果显示,磁学比值参数(如Xpedo/X0、Xfd/HIRM、Xfd%和XARM/SIRM)彼此之间存在显著线性或指数/对数相关关系,它们对成土强度指示明确,可以在一定程度上减小或避免磁性矿物背景值差异所产生的误差,与磁化率相比更适宜用于该区的古降水量重建.HIRM主要由碎屑赤铁矿含量控制,HIRM与X0存在明显正相关关系,表明碎屑赤铁矿随原生磁性矿物总体含量的增加(减少)而增加(减少),在粉尘成因磁性矿物中所占比例大致稳定,从而HIRM可以指示源区粉尘通量的变化.锦界剖面的原生和次生磁性矿物浓度均明显低于黄土高原黄土,不同地层X0和Xpedo在磁化率中所占的比例存在较大差异,因此磁化率的环境意义比较复杂,在使用其恢复古气候古环境时需慎重.