Showing papers by "Jan Kramers published in 2006"

Geologic History of the Central Zone of the Limpopo Complex: The West Alldays Area

Abstract: New field, structural, petrologic, and age data show that tectonic styles and metamorphic histories in the West Alldays area of the Central Zone of the Limpopo Complex, southern Africa, can be linked to neo‐Archean and Paleoproterozoic high‐grade (granulite facies) tectonometamorphic events. The styles comprise (1) a regionally developed high‐grade (S2) gneissic fabric that evolved into a regional (D2) system of sheath folds mapped as circular to oval‐shaped structures with lineations plunging steeply to the WSW and (2) a system of mainly N‐S‐trending and north‐verging (D3) shear zones characterized by high‐grade tectonites (“straight gneisses”) with well‐developed (S3) gneissic fabrics. In the West Alldays area, the superimposition of D3 onto the regional D2 fold pattern produced the kilometer‐scale N‐S‐trending Baklykraal shear zone, which, before this study, was mapped as the Baklykraal fold and interpreted as part of the regional D2 fold pattern in the Central Zone. The Paleoproterozoic high‐...

Geochemistry: how well can Pb isotopes date core formation?

Abstract: Arising from: B. J. Wood & A. N. Halliday Nature 437, 1345–1348 (2005); Wood & Halliday reply Timescale and the physics of planetary core formation are essential constraints for models of Earth's accretion and early differentiation. Wood and Halliday1 use the apparent mismatch in core-formation dates determined from tungsten (W) and lead (Pb) chrono-meters to argue for a two-stage core formation, involving an early phase of metal segregation followed by a protracted episode of sulphide melt addition. However, we show here that crust–;mantle Pb isotope systematics do not require diachronous core formation. Our observations indicate that very early (≤ 35 Myr) core formation and planet accretion remain the most plausible scenario.

Metamafic Rocks of the Tana Belt, Kola Peninsula: Isotopic Age and PT Conditions

Abstract: According to [1, 2], mica schists of the Korva Tundra Group (~1.91 Ga [3, 4]) of the Tana Belt were buried at the middle crustal level and later exhumed simultaneously with granulites of the Lapland Complex (~1.91 Ga [5]). The Kandalaksha Group (KG) is composed of higher-temperature rocks [1, 6], but their geodynamic evolution is similar to that of the Korva Tundra Group [2]. Since the KG rocks have not been previously dated in this area, their synchronous metamorphism with Lapland granulites has not been proved. To solve this problem, metamorphic PT conditions and U‐ Pb zircon ages were determined for two KG samples taken in the Pados‐Yavr interfluve. The results are presented in this work. The hypersthene‐biotite plagiogneiss (sample Pal-9) is a fine- to medium-grained light gray blastomylonitic rock with biotite schistosity and vague quartz‐plagioclase gneissosity. This appearance is related to uneven distribution of melanocratic and leucocratic minerals. The schistosity defined by the preferred orientation of biotite coincides with banding. The texture is lepidogranoblastic. Major rock-forming minerals (Qtz and Pl) in equal proportions compose from 50 to 80% of rock. The Bt content varies within 30‐40% in the melanocratic portions and within 5‐10% in the leucocratic portions. Orthopyroxene (10‐15%) is mainly accumulated in the melanocratic bands and lenses. Opx occurs as an accessory mineral in other parts of the rock. However, both forms of Opx lack mineral inclusions. Some large Opx grains are rimmed by Bt flakes. Secondary minerals are nearly absent. The garnet‐clinopyroxene crystalline schist (sample Lap-34) is a medium-grained dark green rock with a lenticular-banded structure and porphyritic heterogranoblastic texture. The rock is similar in appearance to eclogite. However, the Cpx composition does not correspond to omphacite. Its structure is defined by the uneven distribution of Grt and leucocratic minerals. Cpx is the major rock-forming mineral (up to 90%). Garnet accounts for ~10%. Qtz and Pl account for no more than 1‐2%. Locally, Cpx decreases and gives way to amphibole (up to 10‐15%). Ilmenite, rutile, and zircon are accessory minerals. The Grt porphyroblasts are unevenly distributed as intermittent chains, which are aligned with Pl-rich lenses and bands. Scarce inclusions in the Grt are typically small (up to 1 mm) Cpx, which occasionally form chains marking the surface of intergrown garnet grains in a porphyroblast. The representative analyses of equilibrium Cpx, Grt, and Pl are shown in Table 1. Occasionally, Grt contains Pl inclusions (~0.5 mm). In the Hbl-rich domains, Grt contains Hbl inclusions that are compositionally similar to the groundmass hornblende. Hornblende is typically absent or developed as thin (10‐15 µ m) interrupted rims around clinopyroxene. Only locally, Hbl occurs as small xenomorphic grains up to 0.2 mm across. Elongated grains with convoluted boundaries are less common. Xenomorphic Pl grains (up to 0.2‐0.3 mm across) more often occur as a rim in the Cpx-rich granoblastic matrix. The larger (up to 0.5 mm) equant Pl grains form lenticular aggregates from 1 × 10 to 1 × 15 mm in size. The garnet has a constant grossular mole fraction ( ) = 0.28), while the Mg index, X Mg = Mg/(Mg + Fe) decreases by 10 mol % from the core to the rims. Cpx lacks inclusions and occurs as two morphological types. The predominant type is represented by xenomorphic subequant or slightly elongated polygonal grains up to 0.5‐0.7 µ m across. The second type is represented by large (up to 5 × 4 mm across) weakly XCa