Gradients,Property Templates,and Land Use Change

The scarcity of historical individual-level data makes understanding historic land use decisions and the influence of physiochemical gradients on these decisions difficult. Here, we present several measures of early property agricultural quality based on commonly available data, including elevation and soil type. These analyses demonstrate the influence of physiochemical gradients on initial land division patterns in the Gywnns Falls watershed (Baltimore, Maryland). Moreover, we examine the influence of the template created by early property decisions on continuing human-driven landscape dynamics. This influence is illustrated by 1900-era forest cover patterns, park locations, and modern transportation networks.     

中国晚白垩世陆相红层与恐龙蛋化石群序列及其地层学意义

中国的恐龙蛋化石埋藏丰富,分布广泛。迄今为止,已报道有14个省(区)发现恐龙蛋化石。尤其在浙江天台盆地、山东莱阳盆地、广东南雄盆地以及河南西峡和淅川盆地等晚白垩世陆相红层中发现大量的恐龙蛋化石,同时也发现大量共生的恐龙等脊椎动物化石。中国恐龙蛋化石具有数量大、种类多、保存好、分布广、时代跨度大等特点,因此恐龙蛋可以作为中国白垩纪陆相地层划分与对比的重要化石之一。本文在建立天台盆地赖家组和赤城山组的岩石地层、年代地层和恐龙蛋生物地层层序框架的基础上,重点对比讨论了我国几个主要含恐龙蛋沉积盆地的恐龙蛋类群组合序列及其对应的地层关系。同时,对我国周边相邻地区如蒙古、韩国和印度以及其他各个大陆发现的恐龙蛋化石进行了初步对比讨论。天台恐龙蛋化石群与我国其他几个主要的恐龙蛋化石群之间存在一定的差距。相比较而言,天台恐龙蛋化石群和南雄恐龙蛋化石群组合特征鲜明,前者主要以网形蛋类和蜂窝蛋类为主,后者以长形蛋类为优势类群,结合天台盆地陆相红层中多个凝灰岩夹层获得的98～91Ma和南雄盆地67Ma的同位素年龄,据此,天台恐龙蛋化石群代表了晚白垩世早期的恐龙蛋组合,而南雄恐龙蛋化石群则代表了晚白垩世晚期的恐龙蛋组合。天台恐龙蛋化石群与莱阳恐龙蛋化石群明显不同,莱阳盆地的主要蛋化石类型——圆形蛋类和椭圆形蛋类在天台盆地至今没有被发现。但莱阳恐龙蛋化石群与南雄恐龙蛋化石群比较相似,如莱阳盆地金刚口组以椭圆形蛋类为主,含有少量的长形蛋类,而南雄盆地以长形蛋类为主,含有少量的椭圆形蛋类,其中金刚口椭圆形蛋、薄皮椭圆形蛋、长形长形蛋和安氏长形蛋等2属4种蛋化石在两个盆地均有发现。此外,在莱阳盆地王氏群中下部的将军顶组还发现了相对原始的网形蛋类。综合以上分析,莱阳恐龙蛋化石群大致介于晚白垩世早期的天台恐龙蛋化石群和晚白垩世晚期的南雄恐龙蛋化石群之间,更接近于南雄恐龙蛋化石群,时代应为晚白垩世中晚期。天台恐龙蛋化石群与西峡恐龙蛋化石群非常相似,在科一级分类单元中,至少有巨型长形蛋类、蜂窝蛋类和棱柱形蛋类等6个蛋科在两个盆地中均有发现,尤其是巨型长形蛋科仅在天台和西峡两个盆地中有化石记录,其中西峡巨型长形蛋这一蛋种同时产于西峡盆地走马岗组、赵营组和天台盆地的赤城山组。西峡盆地发现的其他一些蛋化石也可以归入到天台盆地新建立的一些蛋化石类型中。据此推断,走马岗组和赵营组与赤城山组的层位可能大致相当。虽然我们对西峡盆地已报道的部分恐龙蛋化石进行了初步厘定,但鉴于西峡盆地大量恐龙蛋化石的形态学和分类学尚存在较多的问题,两盆地的详细对比还有待于进一步的研究。天台恐龙蛋化石群与淅川盆地的恐龙蛋化石组合差别很大,相应的淅川盆地的恐龙蛋类群与相邻的西峡盆地的差别也较大,此外,在淅川盆地高沟组发现有树枝蛋类,这一类蛋化石在天台盆地未发现,据此推测淅川盆地的时代应晚于天台盆地和西峡盆地。相比较而言,淅川盆地的蛋化石组合与莱阳和南雄恐龙蛋化石群有一定的相似性,都含有长形蛋类、圆形蛋类和椭圆形蛋类等,因而,淅川盆地的马家村组和寺沟组的层位被认为可能与莱阳盆地的金刚口组和南雄盆地的坪岭组相当。综上分析,我们可以初步建立我国晚白垩世恐龙蛋化石群的组合序列及其对应的地层层序和时代框架,从下至上至少包含了4个恐龙蛋化石群:1)天台恐龙蛋化石群,赋存层位为天台群中上部的赖家组和赤城山组,时代约为晚白垩世早期(塞诺曼期土伦期);2)西峡恐龙蛋化石群,赋存层位为走马岗组、赵营组和六爷庙组,晚白垩世早中期(土伦期桑顿期);3)莱阳恐龙蛋化石群,赋存层位为王氏群中上部的将军顶组和金刚口组,晚白垩世中晚期(科尼亚克期坎潘期);4)南雄恐龙蛋化石群,赋存层位为南雄群园圃组和坪岭组,晚白垩世晚期(坎潘期马斯特里赫特期)。此外,随着研究工作的继续和深入,有望建立起包括早白垩世恐龙蛋化石群在内的白垩纪恐龙蛋化石群的组合特征序列,为进一步划分与对比我国白垩纪地层提供更多的基础资料。     

Tectonic settings of porphyry Cu–Mo–Au deposits in the Himalayan–Tibetan orogen,East Tethys

Porphyry Cu (Mo–Au) deposits in the Himalayan–Tibetan orogen formed during the Late Triassic, Early Cretaceous, Eocene, Oligocene, and Miocene and can be classified into different metallogenic belts according to their petrologic features, mineralization ages, and tectonic settings. A close spatial relationship to regional strike–slip faults is evident in all five belts. Porphyry Cu (Mo–Au) deposits exist in a wide range of tectonic environments, including island arc, syn-collision, post-collisional convergence, and continental-transform plate boundaries.

Porphyry Cu deposits cluster in the southernmost part of the Yidun–Zhongdian Belt, along the N–S-trending Gaze River dextral strike–slip fault. Porphyry Cu deposits in the Lijiang–Jinping Belt lie along the Ailaoshan–Red River continental–transform shear zone and the associated strike–slip faults. The Yulong–Malasongduo porphyry belt is controlled by the Cesuo Fault, a NNW-trending regional dextral transcurrent fault that is associated with Palaeogene westward continental oblique subduction along the Jinsha suture. In the Gangdis Belt, Miocene porphyry Cu deposits are localized along N–S-trending normal faults, which were produced by transpression within the regional NW–SE-trending Karakoram–Jiali fault zone (KJFZ). A close spatial relationship between porphyry Cu deposits and strike–slip faults also exists for the Bangong–Nujiang Belt.     

Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

Halogen signatures of biotites from the Maher-Abad porphyry copper deposit,Iran: characterization of volatiles in syn- to post-magmatic hydrothermal fluids

Copper and gold mineralization in the Maher-Abad area, eastern Iran, is closely related to multiple episodes of emplacement of a late Eocene granodiorite into a quartz-monzonitic stock and andesitic volcaniclastic rocks. Hypogene and supergene porphyry Cu–Au mineralization occurred within the porphyritic granodiorite and quartz-monzonite host rocks extensively altered into dominantly potassic, propylitic, phyllic, and argillic assemblages. Temperature and pressure estimates using the plagioclase–hornblende thermometer and Al-in-hornblende barometer indicate that the granodiorite intruded at 758 ± 10°C and 1.4 ± 0.2 kbar.

Biotites from the alteration zones have more variable Al^IV than those in the fresh granodiorite, but nearly all are close to the ideal phlogopite composition. Biotite compositions display an increase in Al₂O₃, FeO, TiO₂, and Cl, but a decrease in SiO₂ and F, from the porphyritic granodiorite and potassic to the transitional phyllic alteration zones. Biotite from the potassic zone (X _phl?=?0.63–0.67) possesses a moderate F content (0.53 to 0.82 wt.%) that is significantly higher than that in the phyllic zone (0.22 to 0.38 wt.%), exhibiting a positive correlation with X _Mg and negative correlation with Cl.

With a decrease in the temperature, log (fH₂O/fHF) and log (fH₂O/fHCl) values calculated for fluids equilibrated with biotite increase progressively from the granodiorite through the potassic to the phyllic zones, whereas log (fHF/fHCl) shifts towards more negative values. Fugacity ratio trends in the Maher-Abad porphyry copper deposit are quite similar to those of other porphyry copper systems. The decrease in halogen content of hydrothermal fluids towards outer parts of the deposits reflects an increase in the degree of mixing between magmatic fluid and meteoric water.     

Plio-Quaternary development of the Baklan–Dinar graben: implications for cross-graben formation in SW Turkey

In the Dnepr-Donets trough, salt domes having their origin in Devonian rocks were formed by four stages of cyclic tectonic activity from Late Devonian through Cretaceous. Three types of domes were formed: open piercement core, crypto-diapir, and crypto-diapiroid. - M. Russell.     

Geochronologic and isotope geochemical constraints on magmatism and associated W–Mo mineralization of the Jitoushan W–Mo deposit,middle–lower Yangtze Valley

The Jitoushan W–Mo ore body is a typical skarn-type deposit with the potential for porphyry Mo mineralization at depth. As it is newly discovered, only a few studies have been conducted on the geochronology and ore genesis of this deposit. The ore district consists of Cambrian to Silurian sedimentary and low-grade metasedimentary strata, intruded by granodiorite, diorite porphyry, granite porphyry, and quartz porphyry. Skarn W–Mo ore bodies are hosted in the contact zone between the granodiorite and Cambrian limestone strata. Within the granodiorite near the contact zone, quartz vein type and disseminated sulphide mineralization are well developed. The Mo-bearing granite porphyry has been traced at depth by drilling. Our results reveal two discrete magmatic events at ca. 138 and ca. 127 Ma in the study area. The molybdenite Re–Os isochronal age of 136.6 ± 1.5 million years is consistent with the first magmatic event. The zircon Hf isotope (?_Hf(t) =??12.55?3.91), sulphide isotopes (δ³⁴S = 3.32–5.59‰), and Re content of molybdenite (Re_content = 6.424–19.07 μg) indicate that the ore-forming materials were mainly derived from the deep crust. The regional tectonic system switched from a Late Jurassic transpressive regime to an earliest Cretaceous extensional regime at ca. 145 Ma, and at ca. 138 Ma, the Jitoushan W–Mo deposit formed in an extensional setting.     

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam

The location of the suture zone between the South China and Indochina blocks in northwest Vietnam has been under debate for decades. Generally, the boundary between these blocks has been placed along (1) the Ailaoshan–Red River zone or (2) the Song Ma zone. The Sin Quyen Formation, lying between these zones, was previously regarded as a Palaeo- and Mesoproterozoic sequence. It comprises its provenance and tectonic affinity. We analysed detrital zircons from two paragneisses and one migmatite of the Sin Quyen Formation employing laser ablation inductively coupled plasma mass spectrometry U–Pb dating techniques. U–Pb ages of these zircons show three main periods of zircon formation: ～2.7–3.0, ～2.2–2.5, and ～1.8 Ga, suggesting that Sin Quyen rocks were mainly derived from Palaeoproterozoic and Archaean basement units. Inasmuch as the South China basement comprises rocks of similar ages, we conclude that the Sin Quyen Formation belongs to that block. Our new data strengthen the view that the suture between the South China and Indochina blocks is located within the Song Ma zone. In addition, zircons with U–Pb ages >3.0 thousand million years represent the oldest minerals reported in northwest Vietnam so far, indicating the existence of Mesoarchaean crustal remnants in this region.     

Zeolite Parageneses in the North Atlantic Igneous Province: Implications for Geotectonics and Groundwater Quality of Basaltic Crust

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth's crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volumes, high cation-exchange capacities, and reversible dehydration. These properties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships that provide sensitive indicators of physicochemical conditions in the crust. Observations of zeolite occurrence in Tertiary basaltic lavas in the North Atlantic region indicate that individual zeolite minerals are distributed in distinct, depth-controlled zones that parallel the paleosurface of the plateau basalts and transgress the lava stratigraphy. The zeolite zones are interpreted to have formed at the end of burial metamorphism of the lavas. Relative timing relations between various mineral parageneses and crustal-scale deformal features indicate that the minerals indicative of the zeolite zones formed within 1 million years after cessation of volcanism. Empirical correlation between the depth distribution of zeolite zones and the temperatures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Similarly, the orientations of zeolite zones can be used to distinguish synvolcanic and post-volcanic crustal deformation. Because zeolites that characterize the individual zones display different ion-exchange selectivities for various cations, reactions between groundwaters and zeolites in basaltic aquifers can result in depth-controlled zones where individual elements are concentrated in the crust. This is established for Sr, which is concentrated by at least an order of magnitude in heulandite, resulting in an overall Sr enrichment of lavas in the heulandite-stilbite zeolite zone.     

Petrogenesis of the Palaeoproterozoic Xishankou pluton,northern Tarim block,northwest China: implications for assembly of the supercontinent Columbia

The Tarim block, one of the largest cratons in China, records an important part of the Proterozoic crustal evolution of the Earth. Many previous studies have focused on the Neoproterozoic magmatism and tectonic evolution of this block in relation to the break-up of Rodinia, although relatively little is known about its earlier tectono-magmatic history. In this article, we present detailed petrographic, geochronologic, whole-rock geochemical, and in situ zircon Hf isotope data for the pre-Neoproterozoic Xishankou granitoid pluton (XBP), one of several blue quartz-bearing granitoid intrusions well exposed in the Quruqtagh area, and discuss these intrusions in terms of their tectonic environment. Zircon LA-ICP-MS dating indicates that gneissic quartz diorite and granodiorite of the XBP crystallized at 1934 ± 13 and 1944 ± 19 Ma, respectively. Both underwent metamorphism essentially coeval with emplacement, a time that is compatible with the globally distributed 2.1–1.8 Ga crustal amalgamation during formation of the supercontinent Columbia. Petrographic and geochemical evidence suggest that the XBP is a continental-arc-type granite and may have been generated by the partial melting of Archaean thickened lower crust; this would suggest that the northern Tarim block was a continental-type arc at ca. 1940 Ma. Our new data, together with previous regional geological studies, indicate that a series of Palaeoproterozoic (ca. 2.0–1.8 Ga) tectono-magmatic events occurred in the northern Tarim attending the assembly of Columbia.