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Precambrian Research, 2009, 168(3-4):
247-258
The Neoproterozoic Quruqtagh Group
in the Tarim Block, NW China, contains
multiple diamictites in the Bayisi, Altungol,
Tereeken, and Hankalchough
formations. These diamictites may represent
three or possibly four discrete
glaciations, although evidence for a glacial
origin of the Bayisi and Altungol
diamictite is ambiguous. To constrain
their age and duration, we dated three
volcanic beds (V1, V2, and V3) in the
Quruqtagh Group using the SHRIMP
(sensitive high-resolution ion microprobe)
zircon U–Pb method. Volcanic
bed V1 near the base of the Bayisi
diamictite yields a 740 ± 7 Ma age, volcanic
bed V2 near the top of the Bayisi
Formation gives a 725 ± 10 Ma age, and
volcanic bed V3 between the Tereeken
and Hankalchough diamictites yields a
615 ± 6 Ma age. V1 and V2 have overlapping
ages, and together these dates
suggest that the Bayisi diamictite was
deposited at around 730 Ma. The
Tereeken and Altungol diamictites were
deposited between 725 ± 10 Ma and
615 ± 6 Ma, and the Hankalchough
diamictite between 615 ± 6 Ma and
542 Ma (i.e., the Neoproterozoic–
Cambrian transition). These dates and
previously published chemostratigraphic
data are consistent with (but doe not require)
the correlation of the Tereeken
and Hankalchough diamictites with the
635 Ma Nantuo and 582 Ma Gaskiers
glaciations, respectively. However, the
new dates are inconsistent with a single
and globally synchronous Sturtian glaciation
that occurred in the pre-Nantuo
Neoproterozoic Era. Instead, currently
available data necessitate that either
multiple glaciations occurred, or a globally
diachronous glacial event developed
during a protracted period between
750 Ma and 650 Ma.
2010010526
巴 西 São Francisco 盆 地 Vazante 群
上 部 碳 酸 盐 岩 记 录 的 中 元 古 代 晚 期 海
洋 氧 化 还 原 条 件 : 来 自 稳 定 同 位 素 和
稀 土 元 素 的 证 据 = Oceanic redox conditions
in the Late Mesoproterozoic recorded
in the upper Vazante Group carbonates
of São Francisco Basin, Brazil:
Evidence from stable isotopes and REEs.
( 英 文 ). Azmy Karem; Sylvester P;
Oliveira T F. Precambrian Research,
2009, 168(3-4): 259-270
The Vazante Group consists of a late
Mesoproterozoic (1.0–1.1 Ga) carbonate-dominated
marine platform sequence
in east-central Brazil. The upper part of
the sequence consists of a glaciomarine
diamictite unit overlain by a cap carbonate.
The δ 13 C profile of the upper
Vazante shows significant negative
plunges, one preglacial (drop of 5.5‰
VPDB) and two postglacial (drops of 9
and 5‰VPDB, respectively). The C-
isotope plunge in the preglacial carbonates
is correlated with low Th/U ratios
(0.1–1.4) and a negative Ce/Ce* shift
(0.4), suggesting deposition under relative
reducing conditions. In contrast, the
C-isotope plunges in the postglacial carbonates
are associated with high Th/U
ratios (>2) and positive Ce/Ce* shifts
(up to 1.5), thus reflecting oxidizing
conditions. Variations in the redox conditions
of the late Mesoproterozoic
ocean, reflected by changes in the Th/U
and Ce/Ce* ratios, are likely attributable
to a combination of both global and local
climatic and oceanographic changes,
similar to what has been inferred for the
Neoproterozoic oceans.
2010010527
古 元 古 代 晚 期 裂 陷 盆 地 的 Ar-Ar 和
U-Pb 地 质 年 龄 : 对 与 加 拿 大
Nunavut 地 区 Churchill 省 西 部 哈 德
森 造 山 运 动 具 有 成 因 联 系 的 支 持 =
Ar-Ar and U-Pb geochronology of a late
Paleoproterozoic rift basin: Support for a
genetic link with Hudsonian orogenesis,
157